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I Thought This Was Obvious!

02/03/2013 7:16 PM

Sometimes when you think something is a no brainer...you're left astounded by what some people come up with. I thought this particular design problem (if you even want to call it that!) was literally a 5 sec decision making process. It turns out that not everyone sees it right away. So...I thought I'd post it and see how you guys do.

A simple ladder frame dolly will support a 30000 lbs load (container). Ladder frame is square hollow structural section. The container's center of gravity is somewhere within a 12" dia circle centered on the frame. The frame will be towed (very low speed approx 2 mph) over uneven pavement on a regular basis. Customer wants standard non-spring loaded swivelling casters. The casters can operate safely at the manufacturers stated max working load capacity (whatever that may be)

Question: What is the minimum manufacturer's stated working load capacity required for these casters?...and why?

Oh, and for those wearing smarty pants...yes I know there's nothing to support the load where the arrow is pointing. The load is a container and sits on on the perimeter framing. The question is about caster sizing and nothing to do with the frame. If you need more (which I seriously hope you don't !!), please don't hesitate to ask.

Please get the right answer...I beg you!

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#1

Re: I thought this was obvious!

02/03/2013 7:45 PM

I would think the wheels should be rated for the full weight each...unless the load is to be placed and removed by crane, and the load sits balanced at all times, then 25% of the load....

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#54
In reply to #1

Re: I thought this was obvious!

02/04/2013 4:52 PM

30,000 lb.
Material Cart

6' x 16' deck made with 1/4" diamond plate steel, stake pockets, rugged hitch plates and remote grease fittings for axle bearings.

http://www.sperlingrailway.com/RailwayProducts.php?30-000-Pound-Carts-25

http://www.sperlingrailway.com/RailwayProductsLine.php?On-Track-Carts-2

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#55
In reply to #54

Re: I thought this was obvious!

02/04/2013 9:36 PM

so the specs given are:

30,000 lb. work capacity.
- 16' X 6' deck with 1/4" diamond plate steel.
- 16" hardened cast steel wheels.
- 2 15/16" axles with pillow block bearings.
- 3/4" X 5 1/2" hitch plates both ends.
- Remote grease fittings.
- Stake pockets.

Whats that tell us? I think someone with some more experience could calculate a rough estimate by the axle size as to what the rating per caster is.

Anyone?

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#76
In reply to #55

Re: I thought this was obvious!

02/05/2013 8:11 AM

Ok here's one with specs....for pavement

King 20ft container trailer 15 ton capacity.
Heavy duty used by the military, choice of units in stock all in excellent condition

Max design gross trailer weight: 20340 kg`s
Max design weight per axle 10170 kg`s
Tyres 265-70R19.5

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#129
In reply to #76

Re: I thought this was obvious!

02/05/2013 10:37 PM

So half the weight is supported by each axle!

Compared to the problem at hand and as per answer in 38# (partially) half of the rating per caster is not to far off.

Good day!

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#56
In reply to #54

Re: I thought this was obvious!

02/04/2013 11:03 PM

Good answer! It is less hassle and less liability (and probably less cost!) to purchase a complete cart from someone in the business of designing and making these beasts!

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#2

Re: I thought this was obvious!

02/03/2013 7:58 PM

If the diagram is to scale, I would think 10,000lbs.

I don't know if there are standard capacities so I figure that is the next up from 7,500 which is not quite enough.

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#3

Re: I thought this was obvious!

02/03/2013 7:59 PM

My experience, I made a set of four (4) of four (4) casters in each set for equipment dollies. And from my experience, it seems the load is rated for static. Because I always had an extra set of casters to replace them after I snap them off when I hit an obstruction...... And the obstruction can be a cement seam of the floor.

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#4

Re: I thought this was obvious!

02/03/2013 8:02 PM

Do you really mean minimum stated working load (which will be just the weight of the unloaded dolly caster) or am I misreading this.

I would have said the obvious, but am not sure about the impact loading of the loaded-down casters due to the uneven surface, although surely you would buy a caster with a load safety margin.

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#5

Re: I Thought This was Obvious!

02/03/2013 8:17 PM

I forgot weight of dolly. 2000 lb.

For Jack, by minimum I mean with the container on the dolly.Ignore impact loads. That's impossible to predict. Mfg caster load ratings already have safety factors built in so stated working loads are fine ie, if the caster says 10000 lb max then you can use it up to 10000 lb with no worries.

For the other guys, just base it on the info I provided. Nevermind loading and unloading the container with crane, etc.

I'm starting to cry now.

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#6

Re: I Thought This was Obvious!

02/03/2013 8:25 PM

Given uneven ground its entirely possible to momentarily have up to 1/2 of the static loading on any one single caster so from that I would suggest going with a minimal rating of 16,000 #'s per caster.

Realistically however given traveling loading and the like even with a safe overload rating I would probably step up to a 20,000 # rated cater for each corner.

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#34
In reply to #6

Re: I Thought This was Obvious!

02/04/2013 12:01 AM

Due to the fact the casters are unsprung and diagonally offset through the centre of gravity (approximately!), I'd give this a GA. When travelling across uneven pavement there will likely be times that it is on a plane of three contact points as Redfred mentions in #7, and can teeter across the two diagonally situated casters that are in contact with the ground to reverse the unloaded caster with the diagonally opposite loaded one. This could potentially be for anywhere from a fraction of a second up, depending on speed, actual load positioning etc. So you need to spec the casters for 1/2 load, or 16,000 lbs as you say the #'s include safety factors including impact loads as it transfers contact points.

I'd hate to tow it @ 30,000 lbs with 4 free swivelling casters tho!!

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#40
In reply to #6

Re: I Thought This was Obvious!

02/04/2013 4:50 AM

On rough ground the load could momentarily be on a single caster. Nothing magic about two being in contact with the ground where a random surface is concerned, unless you can absolutely guarantee the surface will at all times be merely uneven. I would use 30,000-lb rated casters if rough ground is anticipated, particularly if that load is worth a lot more than those casters. Good chance of that being true at least some of the time.

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#7

Re: I Thought This was Obvious!

02/03/2013 8:28 PM

As a non-mechanical engineer I'd guess that since three points define a plane three wheels will always be in contact with the ground. So without any added safety margin the minimum load capacity of each wheel should be rated for at least 10,000 lbs.

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#60
In reply to #7

Re: I Thought This was Obvious!

02/05/2013 12:02 AM

On an uneven surface it will rock over 2 diagonal corners, so it will not have at least three, it will have at least two.

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#73
In reply to #7

Re: I Thought This was Obvious!

02/05/2013 7:31 AM

I totally agree. GA from me. If the wheels have pneumatic tyres it could be down graded to 7500 lbs each.

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#99
In reply to #7

Re: I Thought This was Obvious!

02/05/2013 1:47 PM

You probably remember sitting in a restaurant, rocking on your four-legged chair or playing with the table as it rocks. Because the table is roughly balanced on two points, the third point is very lightly loaded, making the table very easy to rock. Often, even shifting the beer pitcher all the way to one corner is not enough to stabilize the table -- it still rocks on two legs. When smoking was more common, the usual way to fix this situation was with a match book under one leg.

(You may have noticed that this problem is more likely with a pedestal base, in which the support points are relatively rigid. With four corner legs, the table itself twists enough to allow all four legs to be in contact with the floor -- it is a very rare home dining table that rocks, for example.)

TerraMan is looking at this in a grossly oversimplified way, and he is assuming (incorrectly) that the frame/container combo is rigid. Ladder frames are notoriously weak in torsion*, as are containers with open tops. (Compare a shoe box in torsion with and without its lid.) In practice, the load will be close to evenly distributed among the wheels, because the frame and container are easily twisted under a heavy load. The load at any wheel will be roughly 1/4 the total load, plus or minus the torsion load contributed by twisting the frame and container.

So your answer is probably among the best, in a practical sense, assuming typical industrial floor unevenness. If the unevenness is outside that norm, then it (and the floor material) must of course be specified, which TerraMan failed to do.

The center of gravity of the combo is within a circle that is apparently quite small relative to the size of the frame. So in (incorrectly applied) theory, this dolly has the potential for rocking on two points, just like a restaurant table, because the load is approximately balanced along a line between two diagonal wheels.

But Tornado is correct that the size of the CG circle relative to frame size matters, (in the theoretical simplified analysis TerraMan seems to have in mind.) If Terra Man is looking for something glaringly obvious, it is that the frame size is not specified. So perhaps the CG can be within the circle and be 2/3 of the way across the diagonal between two wheels (if the frame is very small). In that case, then one caster receives 21,333 lb and the other 10,667. Given that the casters have two wheels each, then the engineer would need a caster that can accept 21,333 on a single wheel, meaning for some manufacturers, a caster rated at 42,666lb: 45,000 lb to be safe.

*Look at a pickup truck parked with one wheel up on a curb, etc. The twist between bed and cabin can be a couple inches measured out at the cab side.

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#102
In reply to #99

Re: I Thought This was Obvious!

02/05/2013 2:26 PM

I completely agree. I understood the rocking table analogy. Napkins or drink coasters have become the shim standard today to level a wobbly table. I do wonder about the two point condition evenly distributing the load across two points. When all of the forces are matched then no motion (wobbling) should start to happen. This is my "edge" result for a coin toss. I particularly wonder about a two point balancing act with the magnitude of the forces being discussed. When one considers the likely twisting and sagging of the dolly with this magnitude the most likely long term added stress condition for these casters seemed to be a three point support of the dolly.

The CG region specification of within a 12 inch circle is a useless specification without knowing anything about the dolly dimension. It would've been more useful to know the percentage area of the CG versus the dolly area instead of just one value. The drawing did imply a small percentage area but we've all been burned by such an assumption before.

It is also important in a real world application to know the height of the CG above the dolly and any possible motion within the reference frame of the dolly. This will make a significant deviation from a static condition as things change in speed. Not to mention the required added vectors to achieve this presumed minimal velocity.

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#142
In reply to #7

Re: I Thought This was Obvious!

02/06/2013 8:32 AM

I don't think that follows. It's true that 3 points define a plane, but either of the 2 pairs of diagonally opposite wheels take the full load between them. Although one of the other wheels may be in contact with the pavement, the load on it is negligible.

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#147
In reply to #142

Re: I Thought This was Obvious!

02/06/2013 1:05 PM

I don't think that follows. It's true that 3 points define a plane, but either of the 2 pairs of diagonally opposite wheels take the full load between them, assuming the CG is very close to centered.

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#151
In reply to #147

Re: I Thought This was Obvious!

02/06/2013 3:20 PM

......assuming the CG is very close to centered. Yes, and that was stated in the original post.

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#152
In reply to #151

Re: I Thought This was Obvious!

02/06/2013 4:00 PM

Yes, and that was stated in the original post.


It was implied. But the ratio of CG circle spread to wheel spread was not given.
In practice, when loaded to 30,000 lb, all four casters (with their eight wheels) will be in contact with the floor, assuming floors appropriate for the sort of caster drawn. (Neither containers nor ladder frames are perfectly rigid.) At some lighter load, three wheels will be in contact, with one taking perhaps 10% of the total load, depending upon the CG location relative to the frame size. At a very light load, with the cg very close to the line between two casters, two wheels will take the vast majority of the load, but will only instantaneously take all the load.

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#8

Re: I Thought This was Obvious!

02/03/2013 8:44 PM

We don't know how large the frame is compared with the 12" circle, so we don't know how much the load could concentrate onto any one caster (a small to moderate fraction). After also adding the frame weight, I would bump Redfred's answer up to 12,000 lb per caster.

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#11
In reply to #8

Re: I Thought This was Obvious!

02/03/2013 9:03 PM

"We don't know how large the frame is compared with the 12" circle..."

See OP's last paragraph..."The load is a container and sits on on the perimeter framing"

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#12
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Re: I Thought This was Obvious!

02/03/2013 9:08 PM

Irrelevant; if the COG can be anywhere off center in the 12" circle, then the casters will be unequally loaded. How much does depend on how large the frame is compared to the circle.

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#15
In reply to #12

Re: I Thought This was Obvious!

02/03/2013 9:16 PM

"if the COG can be anywhere off center in the 12" circle, then the casters will be unequally loaded"

That's correct.

"How much does depend on how large the frame is compared to the circle"

No it does not. You will see why later and I'm not "Off-Topic" in #11 in case that was you!

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#17
In reply to #15

Re: I Thought This was Obvious!

02/03/2013 9:20 PM

If the frame is only as large as the circle, then the load could all go to just one wheel. I wait with bated breath for any demon-stration otherwise.

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#20
In reply to #17

Re: I Thought This was Obvious!

02/03/2013 9:33 PM

"If the frame is only as large as the circle,.."

What??...It's 12" in diameter. You could only fit 1 caster if it was that big so yeah it would all go to 1 caster but...that's not what's shown in the picture.

You're losing me.

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#22
In reply to #20

Re: I Thought This was Obvious!

02/03/2013 9:40 PM

Well, that was easy.

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#23
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Re: I Thought This was Obvious!

02/03/2013 9:48 PM

You're wishy washy. You went from 12000 lbs to 17000 lbs. Why? I'm guessing because you realized something. So tell me what it is.

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#24
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Re: I Thought This was Obvious!

02/03/2013 9:49 PM

I already gave reasons. Read them, especially the latter.

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#25
In reply to #24

Re: I Thought This was Obvious!

02/03/2013 10:06 PM

By later do you mean #14?

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#26
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Re: I Thought This was Obvious!

02/03/2013 10:18 PM

Of course; I mentioned the 17,000 after the 12,000 (hence latter as well as later).

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#27
In reply to #26

Re: I Thought This was Obvious!

02/03/2013 10:28 PM

Great! Then why were you arguing with me about 12" frame example and all the load going to 1 caster? Your answer of 17000 lb clearly required a different train a thought, no?

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#28
In reply to #27

Re: I Thought This was Obvious!

02/03/2013 10:45 PM

So? Both trains of thought are relevant. And interrelated, not necessarily different.

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#32
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Re: I Thought This was Obvious!

02/03/2013 11:28 PM

CR4 ADMIN: Modified Post

Abuse/Attack: This post was deleted because it was an attack on another user. Please review the CR4 Site FAQ and the CR4 Rules of Conduct.

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#33
In reply to #32

Re: I Thought This was Obvious!

02/03/2013 11:44 PM

Where exactly did that come from?

What's with the personal attack?... and for what reason?. This is supposed to be a fun brain teaser. I haven't attacked you or anybody else.

Dude WTF?

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#35
In reply to #33

Re: I Thought This was Obvious!

02/04/2013 12:21 AM

It was obvious! Well, to some folks anyway.

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#38
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Re: I Thought This was Obvious!

02/04/2013 1:16 AM

Yes Tornado it was, but only to you at #14. You accounted for the fact that the possible 6" offset from center means it's not exactly 16000 lbs per caster. Tcmtech got the correct reasoning first but missed the slight offset thing. Redfred's 3 point contact thing was good but it's where a lot of people stumble. 10000 lbs per caster would apply only if the CG is the same distance from 3 casters where in this case it cannot be. A dolly like this will always have only three casters touching the floor (isostatic system). 2 will share the load with one of them sharing more of it. The 3rd caster sees a small portion of the load.

Then there's Kramarat's comment. I really have no clue where that came from but it answers the question I've had for quite a while now. There is indeed a "club" here at CR4 and you guys do a fantastic job at alienating people who come here voluntarily to contribute knowledge. I guess if you're not retired, 60+, and crotchety, then you're not welcome here.

Message received loud and clear.

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#53
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Re: I Thought This was Obvious!

02/04/2013 4:30 PM

"I guess if you're not retired, 60+, and crotchety then you're not welcome here."

Does two out of three qualify me?

Frankly, as the most vilified member of the "club", I ask that you just slip on your Kevlar Feelings Protector and jump right in.

This forum is very well behaved and administered compared to every other forum I've looked at. (which isn't many, granted)

Stick around, I think you're a good fit, for what it's worth.

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#133
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Re: I Thought This was Obvious!

02/06/2013 12:24 AM

Lyn, I must take this opportunity to respectfully request that you, as a senior ''sarcast'' in residence at CR4, please "mentor" me to become a member in bad standing, of the (unofficial ?) CR4 Country Club for Crotchety Cronies.

I have several promising(?) qualifications:

- I am 66, so that qualifies me as being "old", or at least over 60+, even with "fuzzy math"...

_ I'm still working more than 40 hours per week, so I can not say that I am retired, yet. However, my cheapskate, tightwad, boss has hired a young, smartmouth, upstart engineer to replace me as soon as possible, so I have to expect to be made to retire in the near future, as soon as I finish training him. He thinks he knows it all already, but he's not a registered engineer, yet. I've been a registered Civil Engineer since 1980 but he's already acting like he's my supervisor... However, he is blissfully unaware of what it takes to pass the current registration exam, and I'm not about to tell him... But, he is actually pretty "sharp" (when he's not mouthing-off, that is...) so I do expect him to pass it after two or three tries....... So, I am expecting to get retired in the foreseeable future.,,, And then he'll probably want a big raise, and not get, and look for work elsewhere, while said boss hires another upstart, to be trained from scratch...ha, ha, ha, ha, ha.........

- Lastly, I am not actually crotchety, just yet. (occasionally, cranky, but not actually crotchety...), but I'm wiiling to work dilligently on that under your inspired guidance and direction...

So, in anticipation of becoming fully qualified in relatively short order, I respectfully request that you allow me to become your student and that you become my CR4 sensei... (and yes, I do most humbly apologize for not being a mechanical, or an electrical, engineer, so please overlook that deficiency, won't you/0

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#134
In reply to #133

Re: I Thought This was Obvious!

02/06/2013 1:39 AM

Dear sir

I have been around for a while in CR4 and honestly, that was one of the funniest and most wittingly orchestrated piece of writing I have seen in a long time, at least here, were things get hairy and no barber in sight.

Well done, some happy reader here, Ky.

PS: Trust me, Lyn is going to love this too

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#204
In reply to #134

Re: I Thought This was Obvious!

02/11/2013 11:33 PM

Ky,

I just wanted to specifically thank you for your positive comments. I appreciate them very much.

By the way, it's interesting that you should describe my writing as well-orchestrated. In explanation, I should explain that one of my grandfathers actually was a conductor of a small orchestra in the late thirties for a while. Reportedly, it had strings, and reeds, but no horns... I don't understand it either, especially since all I can play is a tape...

Another thing I don't understand is not the fact that you got 5 GAs for off-topic, but that I did not get a single one for any thing... The "Club's" voice seems to have roared with their silence...

Let me give you a sixth GA.

In any case, if you enjoyed my writing, you could also enjoy my:

''The 12 Days of Engineering - Contest'' (comment #12) and/or

''NZ Quake Collapse Building Design Defficiency'' (comment #9) and/or

''Coastal Home Construction'' (comments #12 and 13)

All of your replies will be similarly appreciated.

In any case, No Faux Shui...

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#64
In reply to #38

Re: I Thought This was Obvious!

02/05/2013 12:56 AM

I guess Im not in the club. Im not over 60, definately not retired, crotchety? Thats a matter of opinion. I have been the target of a few arrows but I dont feel alienated. May I suggest your having a bad day? Im sure that we all have them.

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#66
In reply to #38

Re: I Thought This was Obvious!

02/05/2013 2:24 AM

"There is indeed a "club" here at CR4 and you guys do a fantastic job at alienating people"

I can't speak for crotchety old vilified geezers like lyn (congrats on your new 'Most Vilified' status, btw, and yes, lyn, you finally spelt 'vilified' correctly ), but for me personally? I come here to get my fair share of abuse but nearly always leave disappointed. "Fantastic at alienating people?" Who are you kidding? We're still light years away from certification (not that nobody's tried, but the Admin here are far too vigilant on that front, bless their Red Stars and Black Hearts).

For Real Alienation I'm afraid you'll have to appeal to Facebook. There you'll find lots (and by lots I'm talking millions) of Keyboard Kommandos, Trolls, Haters and meth-lab dropouts with IQs in the high 20's and permanent mob mentalities who'd like nothing better than to keel-haul your hide Just Because. Fantastic? Ha!

Pansy.

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#139
In reply to #66

Re: I Thought This was Obvious!

02/06/2013 7:51 AM

That's because the medium in it's elements, allows for verbal abuse, being so impersonal, clean-out of natural human interaction, such as sight, close proximity, the ability to read body language, and such.

It can only show that informative or verbal is never enough in order to read each other's sensitivities and emotions, and conduct ourselves accordingly.

The were so many times I realised in hind-sight, I offended someone on the web, without real intention to do so, mostly because I couldn't read their body language during the exchange of massages.

As a result I vowed to myself never to be offended emotionally by any type of verbal expression on the web, knowing the offender's side is void of real human feedback - as we really are when meeting face to face, flesh and blood.

Not like I always manage to avoid being hurt, but I always keep the above mentioned in mind when reading text, any text, on the web

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#112
In reply to #38

Re: I Thought This was Obvious!

02/05/2013 3:36 PM

Terraman! I think that you have hit it! Yes I agree there seems to be a good ole boy theme here! out ciders Can't have a chance to pose a good question to spark discussions & get back open discussions without this childish remarks simple known questions. Half of the comments are all from that so called group? THEY THINK!

It's not their Forum when WE the others ask the question,& in that WE are the CR4 sparking what this site was intended for.

I found if they don't know & never had some life's' experiences they say it doesn't happen or how do you know it works, Simple schooling gets them up set & attack.

It's like being back in elementary school with a room full of I- Idiots.

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#114
In reply to #112

Re: I Thought This was Obvious!

02/05/2013 3:47 PM

"It's like being back in elementary school with a room full of I- Idiots."

.

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#116
In reply to #114

Re: I Thought This was Obvious!

02/05/2013 3:55 PM

So eurpium ! do you even know what the picture is in your reply!

Or is this reposting my picture as a joke?

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#117
In reply to #112

Re: I Thought This was Obvious!

02/05/2013 3:57 PM

As someone who's only even known of CR4 for less than a year, seems maybe I'd hardly be considered part of the ole boys club. So I also think I'd be entitled to say I've found out ciders are generally very welcome when they haven't already been in the cider.

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#118
In reply to #117

Re: I Thought This was Obvious!

02/05/2013 4:04 PM

It's simple questions that spark real answers from others! Not feeling that all forums are full of uneducated people with the cider being part of the good ole boy club, NOT Drunks.

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#120
In reply to #118

Re: I Thought This was Obvious!

02/05/2013 4:19 PM

It's simple questions that spark real answers from others!

That is actually a very good remark, very often true. Unfortunately what's simple to one person may not be simple to another- as this thread illustrates very well!- and it really isn't fair to judge others based on that.

When someone insists on posting incomprehensible jargon and continually sticks to there own point that's another thing. I'm not an engineer & there are a couple of OT side threads on here that make my head spin. But they're interesting and educational, and for myself that's why I keep coming back here even though I may not agree with some comments.

So anyone who wants to post simple questions might find things different from if they just repeat their own opinions and then take offense when others tell them to go away. If you're going to dump out a pile of cr4p you can't complain when some gets thrown back at you.

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#164
In reply to #112

Re: I Thought This was Obvious!

02/06/2013 7:08 PM

I posed a question a couple of months ago and was told I should have discussions with you as we were of the same ilk. I'm still researching, but I've come to the conclusion that some of the die hard engineer's who swear by what they were tought in school as immutable 'facts' are in for a rude awakening. Bearden's MEG works and was patented a couple of years ago. Why does it work? I mentioned a video at 'thrivemovement.com' and only one person attempted to review it and was worried about getting 'spammed' and thought there was a required donation of $25 and so it was a 'scam'. I mentioned PJ Kelly's compilation of patents and info and was told 'facts' and if it didn't fit facts...so I research and am currently constructing a project along Don Smith's 'resonant' dipole system. We'll see. None of these nay sayers have done any research of any of those lines. Just can't work, we learned THAT in school.

Sorry about the rant

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#165
In reply to #164

Re: I Thought This was Obvious!

02/06/2013 8:00 PM

"Bearden's MEG works and was patented a couple of years ago."

Had not heard of anything called "MEG", but Google found this, along with the usual list of crackpot over-unity supporters.

http://en.wikipedia.org/wiki/Motionless_electromagnetic_generator

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#176
In reply to #112

Re: I Thought This was Obvious!

02/07/2013 3:22 AM

"...there seems to be a good ole boy theme here! out ciders Can't have a chance to pose"


The Good ol' Boys. Ciders all.


An Out Cider posing with some half-eaten bikkies.

-----

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#44
In reply to #33

Re: I Thought This was Obvious!

02/04/2013 9:05 AM

9:30 PM on Super Bowl Sunday? You have to ask?

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#42
In reply to #17

Re: I Thought This was Obvious!

02/04/2013 7:20 AM

Let be the edges A-B-C-D. Contact on ABC and load on AD or CD. The whole will tilt around the axis AC til D comes to a contact and B goes up. Thus contact at least on 2 wheels.

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#9

Re: I Thought This was Obvious!

02/03/2013 8:49 PM

Frankly I think it's an unsafe design, considering it has to haul 30,000 pounds.

I'd double the number of casters, in case any one should jam or fail.

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#13
In reply to #9

Re: I Thought This was Obvious!

02/03/2013 9:12 PM

This is not about the validity of the design. It's just an example to illustrate the question.

BTW, I've never seen a dolly design like your concept and there are reasons why it's not done that way. I'll explain later. But it is cute!

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#29
In reply to #13

Re: I Thought This was Obvious!

02/03/2013 10:47 PM

It was just an illustration; think of the truck systen on a rail car.

I don't need your explanation.

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#30
In reply to #29

Re: I Thought This was Obvious!

02/03/2013 11:11 PM

I'm familiar with that type of system but that's not what you've illustrated. You showed 2 casters next to each other on a frame. What's missing?...because what you're showing doesn't work.

(don't Google it)

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#45
In reply to #30

Re: I Thought This was Obvious!

02/04/2013 9:16 AM

I've made my point, that it lacks redundancy; the bumpy floor you mentioned could be a problem. Instead of a single dual-wheel caster at each corner like you show, it ought to have something like a railcar truck at each corner. Beyond that, I've lost interest in this.

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#10

Re: I Thought This was Obvious!

02/03/2013 9:02 PM

16,000 pounds each, but two are rigid, non-swivel casters.

Otherwise, how would you tow it?

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#14

Re: I Thought This was Obvious!

02/03/2013 9:13 PM

If one wheel loses contact, the load will be almost equally divided by the two other diagonal wheels. So now we are at 16,000 lb/wheel, plus a bit for off-center allowance. I have personally used 17,000 lb wheels on a project, so I revise my nomination accordingly.

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#16

Re: I Thought This was Obvious!

02/03/2013 9:19 PM

Unless to want this thing to slew all over the place when its being towed, I'd be putting a set of fixed (non swivelling) castors in the middle of each (long) rail. (no cad on this puter so no drawing sorry).

You don't mention if the wheels will have pnuematic or solid tyres. Pnuematic tyres would alleviate some of the issues regarding uneven pavement surfaces as the tyres would provide a degree of compliance where as the solid tyres will cause some load instablity while negotiating the uneven path.

A 13 tonne payload is not to be trifled with, even at 2 mph.. there are OH&S questions to be resolved with this project, you don't mention brakes, you're going to need them.

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#18

Re: I Thought This was Obvious!

02/03/2013 9:26 PM

The casters have swivel locks so you can lock any 2 (for tracking when towing). Fine, I forgot to mention that but...who cares. Make 2 of them rigid. I don't care. It's not about that. It's about how much of a load rating do they need to have.

Sheesh.

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#19
In reply to #18

Re: I Thought This was Obvious!

02/03/2013 9:32 PM

Depends the safety margin dictated by the undisclosed specifications for the carrier.

Or, some other unknown.

This has suddenly become more boring than the game.

<bye>

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#21
In reply to #19

Re: I Thought This was Obvious!

02/03/2013 9:38 PM

Why were you not confident in your first answer in #10?

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#31

Re: I Thought This was Obvious!

02/03/2013 11:23 PM

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#36

Re: I Thought This was Obvious!

02/04/2013 12:35 AM

You're right. It's obvious.

With uneven ground, the load will be on two wheels (diagonally opposite on the frame). If each caster has two wheels, as shown, then a single wheel on each of two casters will sometimes support the load. You'd want 32,000 lb casters in each location, unless you have the caster manufacturers assurance that a single wheel of a caster will take the full load of the caster rating.

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#37

Re: I Thought This was Obvious!

02/04/2013 1:16 AM

You had a similar one already.

Link

Going with this I would say you would ask for a rating of at least half the full load of each caster.

But then what do I know?

If anything fails K Fry will know!

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#39
In reply to #37

Re: I Thought This was Obvious!

02/04/2013 1:25 AM

You had a similar one already

Yes. Many thought it was fun so I thought I'd try another but after this one I learned my lesson.

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#41
In reply to #39

Re: I Thought This was Obvious!

02/04/2013 6:31 AM

So what is the solution then? You know if you do not post it it will not look good for CR4. In the end it is a source for later research. I know you know that. So fill us in and put the grumble to the side.

Or did I miss the solution in the earlier posts?

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#43

Re: I Thought This Was Obvious!

02/04/2013 8:49 AM

I was right. The game was better.

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#46

Re: I Thought This Was Obvious!

02/04/2013 10:26 AM

20000 # over uneven floor. Not all wheels maybe in contact with floor surface at all times. At least two will be the will need to handle the load. The load shared each would need to handle 15000#. Because of the uneven surface a safety margin is also needed. So the 20000# a piece. Would also choose a king pin caster. Their usually more durable.

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#47

Re: I Thought This Was Obvious!

02/04/2013 11:11 AM

Maybe castors are the wrong solution. Maybe a railway/railroad would do a better job at keeping the load moving in the correct direction in a controlled manner and spreading those corner loads from the frame onto a greater ground surface area, especially if it is uneven. Maybe that's the no-brainer.

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#48

Re: I Thought This Was Obvious!

02/04/2013 1:34 PM

OK we have all attempted an answer TerraMan, what is the obvious answer and the reasoning behind it please.

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#49
In reply to #48

Re: I Thought This Was Obvious!

02/04/2013 1:56 PM

The explanation I want to see is what uneven pavement will support sixteen tons of force at two, three or four apparently solid casters without permanent deformation.

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#50
In reply to #49

Re: I Thought This Was Obvious!

02/04/2013 2:12 PM

That's why a never gave an actual figure with the given information.

It may be that the uneven surface shouldn't have been mentioned to simplify the question as this is apparently only a simplified theoretical question not a practical one.

Hopefully TerraMan will answer and explain his posed question before this thread gets out of hand with people assuming things he never intended.

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#52
In reply to #49

Re: I Thought This Was Obvious!

02/04/2013 4:05 PM

Well that depends really if you consider the term caster to only be smaller sized wheels like what a utility cart or production floor dolly uses then yes they would sink right through any thickness of pavement with 8+ tons of weight on them.

However when you get into the 8 ton plus range of casters they tend to resemble a size and shape that looks more like the set of dual tires from a delivery truck or large warehouse forklift which in that case can spread out the weight of 8+ tons without excessive deformation of the underlying surfaces like pavement or hard packed earth.

Granted getting them to do a full 180* swivel in the spot while at maximum load would likely do a bit of surface damage to most pavements!

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#51

Re: I Thought This Was Obvious!

02/04/2013 3:33 PM

TM, you didn't say the client was stupid so I assumed the cart was to run over a smooth and level floor, and in that case, the only problem is the eccentricity of the load center in the 12" circle.I didn't have the weight of the cart but it doesn't change my answer.

If it does run over rough ground, it could be supported, instantaneously, on two casters and not necessarily equally. I have been advised in the past that working capacities are factored on continuous use and that a short term overload is not important.

A sensible client would want spring casters.

Another consideration that could not be approached for lack of information was the stiffness/flexibility of the cart., the two wheel case depends on the cart being rigid, or nearly so.

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#70
In reply to #51

Re: I Thought This Was Obvious!

02/05/2013 4:57 AM

I would assume that the frame has some unavoidable flexibility, however, once loaded with a relatively rigid container the loadings will be the same as if the whole thing is rigid. Hence the maximum static load could be divided between two diagonal castors according to (D" + 6")/2D where D" is half the distance in inches between opposite corner castors. If it makes any material difference take the pivot radius into account as this will move the wheelbase to and fro aroung the center of gravity. But this might be why the 12" circle is there in the beginning.

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#57

Re: I Thought This Was Obvious!

02/04/2013 11:14 PM

I guess Kramarat offended TerraMan enough to chase him away. That's a real pity because I was hoping for an explanation why a towed or static dolly required four casters rated to each handle at least a half instead a third of the load. (Certainly the dolly weight should be included along with a nominal safety margin.) I do not like the balancing act answer that a diagonal pair of wheels might support this large amount of mass. I suspect this will happen about as often as a coin toss produces an edge instead of a heads or tails result. The balancing act would likely just be transitional at best.

But as my earlier answer states, mechanical engineering is not my expertise. I wish to learn why two wheels instead of three should be considered and not to be ridiculed that the answer should be obvious. I've many times explained electrical engineering nuances here to others that do not work with tiny electrical currents. I do not expect most EE to understand the solid state reasons why Johnson and Schottky noise distributions are different. I wouldn't even dare tell a theoretical Physicists that it was obvious why they are different from examining the valence electron probability distributions.

The more I think about this, Kramarat is right.

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#58

Re: I Thought This Was Obvious!

02/04/2013 11:46 PM

15000lb min because as the container is hauled over uneven ground, at times it will be balancing on only two diagonal casters

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#61
In reply to #58

Re: I Thought This Was Obvious!

02/05/2013 12:06 AM

then adding the frame = 16,000 plus impact loading on the uneven ground, something like 25,000lb each would not be an unrealistic loading to choose, although perhaps 30,000 or more would be a wiser choice.......

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#59

Re: I Thought This Was Obvious!

02/04/2013 11:56 PM

I would have thought a non-castoring wheel each side in the centre, rated for half the load and a castoring one each end in the centre and set higher, so that only one castor contacts.

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#62

Re: I Thought This Was Obvious!

02/05/2013 12:11 AM

A 30000 pound load centered within a 12 inch circle. Dolly will support a 30000 pound container. I think its obvious the dolly will not support 30000 pounds in a 12 inch circle no matter what size the casters are. A container sized to the perimiter of the dolly would NOT keep the load in a 12 inch circle

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#63

Re: I Thought This Was Obvious!

02/05/2013 12:41 AM

I'd start by saying force estimates on a load like this are not trivial.

The alloy of the frame, welds, alloys used in the caster bearings, etc. are only the starting point.

The seemingly trivial tidbit of information regarding the uneven surfaces, IMHO, could possibly require a per wheel load rating far exceeding the total mass of the system. The reason is that the diamter of the wheels in relation to the height and shape of the uneven elements of the path this will be pulled on, could result in a step function impact force that would, if it were to be pulled by a tug or truck with sufficient mass, launch your load into the air (a 1/2" launch would represent a lot of upward force on the caster to the load), even possibly shear off the casters or destroy them if they are rated for only the load, however you calculate it to be distributed.

Think of it like this: Boy's wagon with a cat for a load. Pulled by your truck over a 10" curb (vertical side), you probably will shear off the wheels since they are (likely) centered below the top of the curb, no matter what speed you are travelling at, and the cat won't come near you for a week.

Pulled over a 3" curb, supposing the wagon wheels are 8", you will jack that wagon up in the air with a momentary force proportional to the speed of the truck and weight of the cat and wagon combined.

Maybe we could charter someone to buy the boy an new wagon and do an experiment with the speed of the truck vs. how long before the boy sees his cat? 1, 3, 5,7,11, 23, 47mph? Care to try for 60mph, anyone?

I believe we need to know the unevenness of the surface, diameter of the casters, mass of the pulling vehicle, and the maximum speed this will be allowed to travel at.

Then we can calculate the forces from this uneven surface. Next, consider the caster's ratings for both axle loading and maximum impulse load. Two ratings, one for estimating the life of the bearing, and the other to see if the caster will survive the impact loading. I would love to shoot from the hip and give you the obviuos answer, but this is, after all a sue-happy world we live in...

If the caster manufacturer does not rate the important information, you may need to have a sample tested. Think in terms of the weakest link. Make sure the failure you have is not the one that will cause unbearable consequences.

If you were fishing to see if you did indeed have it oversimplified, I would guess that you did, since you did not specify the real characteristics of the uneven surface or the the pulling forces and maximum speed.

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#65
In reply to #63

Re: I Thought This Was Obvious!

02/05/2013 1:49 AM

All highly relevant points, WhiteHorse. However, since TerraMan opened this thread with, "I Thought This Was Obvious", I believe he was looking at the problem too simply. That might explain his reaction to the numerous posts addressing variables that he doesn't think are important.

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#78
In reply to #63

Re: I Thought This Was Obvious!

02/05/2013 10:08 AM

Yes! This is the thought process I was looking for to understand the complexity of safely supporting and moving 16 tons of weight. A 16 ton mass moving at the proposed speed of 2 mph is almost 6000 Joules of kinetic energy. A change in elevation of only 1 inch on 16 tons translates to a potential energy change of this magnitude. So the magnitude of the unspecified elevation changes are not trivial.

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#67

Re: I Thought This Was Obvious!

02/05/2013 4:09 AM

Good lord. Apparently we need to add illiteracy to a number of peoples resumes here.

Directly from the OPs opening post. I see a few certain people here need to pay closer attention to the parts I have in bold and the specific part I have underlined as well.

"A simple ladder frame dolly will support a 30000 lbs load (container). Ladder frame is square hollow structural section. The container's center of gravity is somewhere within a 12" dia circle centered on the frame. The frame will be towed (very low speed approx 2 mph) over uneven pavement on a regular basis. Customer wants standard non-spring loaded swivelling casters. The casters can operate safely at the manufacturers stated max working load capacity (whatever that may be)

Question: What is the minimum manufacturer's stated working load capacity required for these casters?...and why?

Oh, and for those wearing smarty pants...yes I know there's nothing to support the load where the arrow is pointing. The load is a container and sits on on the perimeter framing. The question is about caster sizing and nothing to do with the frame."

Anyone?

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#71
In reply to #67

Re: I Thought This Was Obvious!

02/05/2013 4:59 AM

I am surprised you are surprised !

If you read the answers the majority does NOT answer the original question but only gives suggestions how to do it better.

This is in fact a sigh of under-appreciation of the one who asks for an input with a specific need.

Many do not answer the question but participate only to show how much they know on other subjects.

As for answer the question I would use parts with at least half of the total nominal load since the it could be considered that the COG eccentricity is small with respect to width and so in case that the structure has only 3 contact points with ground the load is near to diagonal and most of it will be taken by 2 parts.

If we consider the COG on the diagonal then one point will be loaded with half of frame weight and with the corresponding part of the container weight considering the fact that the load is eccentric thus nearer to the most loaded point.

If the GOC is for instance on diagonal AC toward C the highest load is on C = Frame/2+ (1+12/AC)*Container weight/2.

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#101
In reply to #71

Re: I Thought This Was Obvious!

02/05/2013 2:12 PM

Nick authoritatively proclaims: "Many do not answer the question but participate only to show how much they know on other subjects."

Ah! Another telepath! I do need to try harder....

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#68

Re: I Thought This Was Obvious!

02/05/2013 4:17 AM

How are you feeling about your 'obvious' answer now?

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#69

Re: I Thought This Was Obvious!

02/05/2013 4:18 AM

Take no chances.

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#72

Re: I Thought This Was Obvious!

02/05/2013 5:40 AM

What is the minimum manufacturer's stated working load capacity required for these casters?

erm......the casters have to support a load of 30000 lbs. Well, you've had all the sensible answers.

What the heck, go for full load on each. The cost increment for stronger casters may not be too bad, and somebody is bound to abuse the poor dolly anyway.

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#193
In reply to #72

Re: I Thought This Was Obvious!

02/08/2013 3:31 PM

Damn I've had to give you a GA...
Only cos I was worried about Dolly...
A sheep isn't just for Sunday you know
Del

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#74

Re: I Thought This Was Obvious!

02/05/2013 7:33 AM

In case you missed it Terraman's answer is at #38.

Terraman, don't stop posting these. You make some good points and bring up real (not theoretical) situations. It shows your experience and your willingness to educate others. These posts are appreciated.

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#75
In reply to #74

Re: I Thought This Was Obvious!

02/05/2013 8:04 AM

Thanks. I replied before reading all the way through. Good to know i was wrong, again.

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#77

Re: I Thought This Was Obvious!

02/05/2013 9:37 AM

I would suggest making each caster able to handle the entire load + the weight of the frame, and any impact that may be introduced while loading and unloading the cart. Also as the cart can be loaded off center as there is no centering devises installed on this cart. With this in mind I'd probably use casters that could handle a minimum of 45000 lbs each. Why? because I would think that each caster will at some point see the entire load and any additional forces of loading,unloading, and/or motion. Keeping a safety factor of 1.5, I would think nothing less than casters able to handle 45000 lbs. each. That would only be my opinion, which has been suggested to be useless by many folks.

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