Trash Rake Design

Resubmit the drawings with larger size, please, we cannot zoom in on those.

What part of the trash rake failed? Is this in harbor, or out in a bay or somewhere?

what broke was the rail, the lower part started to deform as you can see in the picture bellow (hopefully is large enough). So the trash rake only inclines (angles out) on the east side, so after a while, the rail buckled. You will see in the detailed pictures.

It is a hydro plant.

they installed tese plates for support. compare the condition of the rail in this picture with the next one.

The original equipment manufacturer will be interested and will offer solutions, as this improves the design for future builds. It would save a lot of to-ing and fro-ing if the forum could know what the manufacturer has said in response to the telephone calls made to it as a result of this failure of its equipment. Please advise?

Did I see SPAR TX?

It is impossible for the forum to know what a subscriber sees.

The freaking OP knows what I was referring to.

Recommendation: pull your bottom lip up over forehead until further notice.

The only reason I said that, the air conditioner A-coil froze up again to my lab, and it is like a danged sauna in here this morning. Cheers, and I hope you are enjoying your mint julip about now.

What's causing your coil to freeze up? ....or do you know...

I suspect that the gits who serviced our A/C units last were skimping on the Freon.

Well if don't have a leak now, you will if it keeps freezing up....You can check the condenser air temperature, but make sure the evaporator coil is clean and the airflow is good....You should get a 20- 30° temperature rise across the condenser coil....

The brand is Spar Tek. They are located in Oregon, but they are not the manufacturers; they didn't even installed it, they used a contractor and don't have any drawings.

Then you are now embarking truly on a course of reverse-engineering to which you and your company are entitled. Now will be the time to clearly point out the weaknesses of your system, and eliminate them.

I tend to fully agree with Solar Eagle, that your best approach may lie in speeding up the tipping cycle, so that structural loading is reduced.

You also need to decide what the new PM schedule should be for each moving and non-moving part on the trash rake.

Paint, adding reinforcement, replacing worn parts. Think in those terms. but also it is clearly a time for an intervention and upgrade on the controls and program for this unit.

first, thank you so much for all you comments and time.

So:

speed up the times the machine is used to reduce the load? ok, will do.

PM, meaning, preventive maintenance? I will definitely look into that.

I don't know if we can upgrade the controls and/or the program for this unit. I think it can only be controlled manually, which is another problem I am dealing with, because if it could run automatically it would be ideal. or semi auto

there is nothing out there in the world that is a machine that cannot be made to at least minimally function in an automatic mode.

You might even control this trash rake with an Arduino, but you can afford a real industrial PLC from some place like DigiKey, or the like. Some of them come with their own tablet like interface/display screen.

The original equip manufacturer is no longer in the picture. I tried everything I could to find original drawings and to contact them but it was to no avail. It was installed in the 90s.

I would go check out some similar facilities....

In your first picture is that E.T with his arm raised signaling for HELP?

You must be pulling up a lot of weight, do you have an estimate of max weight?.....how often do you rake? Can you speed up your raking intervals? What type of debri are you harvesting? What is the size of your grate? What is the gph flow rate required?

I don't have an estimate of max weight, but I do know that during the season, march-October, they use the rake pretty often so its not that much weight, really. I would say that it can be used as often as every 45 min. We speed the raking intervals based on how dirty it gets. We are harvesting mainly logs and weeds. What do you mean what is the GPH required? I can probably find out what is the GPM of the water going through, if that would help. Grate size is in the picture:

It looks like corrosion+plus weight damage. Was there a lot of rust in the weak/damaged area's.

If it rusted out, then that's a maintenance issue...

..well that's what I want to know.. if the original and repaired design are being replaced.. modified.. why? How many years or months before a repair was made to the inadequate design?

We haven't decided on replacing the units. They might work as they stand for the next little while, that's part of what I am trying to find out. Repair was made this past spring. It seems to be working ok so far. We have being checking for displacement in the rail since the fix but its too early to tell.

If there's rust driven damage to the rail it will strain the system and cause further problems. Keep the rail in top shape and all will be well... With PM

I completely understand how the rail could be damaged over time.

Replace or repair. Done well either will be fine.

I like the support plates. Just try to avoid creating places where water doesn't drain and dry quickly.

Yes, it looks like there is a lot of rust in the damaged area

You might start with the locked rotor capability of the cable lift motor (~150% torque NEMA Design B, higher for others), multiplied by the reduction ratio as seen with the minimum cable wrap on the drum, plus all gearing.

Add to that the weight of the equipment on the rails, to find an upper limit for the applied force to the rail section. If that far exceeds rail section moment, then a load limiter electronic shear pin or mechanical clutch might be a better solution, rather than attempting to power through the unusual condition. Maybe set the rake to clean at shorter intervals, either with automation or schedule. Alarm when the water level differential gets too high, to override the normal procedure.

A small motor current recording module you can dump to a laptop could show you the frequency & magnitude of the upsets, and help provide some direction.

https://www.microdaq.com/supco-logit-current-voltage-data-logger.php

IF your hydroelectric water source is all that trashy, how come you can't clean it up at least partially before the junk/trash gets to the rake? That is what boats are for.

I don't think using a boat would be feasible. Regardless of how trashy the water gets, the trash rake failed and I need your help figuring out why.

thanks again.

first, thank you sooo much for this comment. But can you please make it in lame terms for me.

I should be able to find the weight of the equip on the rails, but the locked rotor capability of the cable lift motor went over my head.

How do I find the rail section moment?

can you tell me more about the load limiter electronic shear pin or mechanical clutch? why would the water level being to high matter?

also, can you please elaborate on: A small motor current recording module you can dump to a laptop could show you the frequency & magnitude of the upsets, and help provide some direction.

thanks again,

What you are looking for is the upper limit of the force applied to the steel. There are some machine designs that require a drive motor torque capability higher than the machine strength, like clarifier rakes, so supplementary mechanical protection is provided, to keep from twisting the rake up like a pretzel.

The locked rotor torque at the shaft of the motor for NEMA Design B is 150% rated, You should find NEMA Design B, C or D on the nameplate. C & D are higher.

Motor torque = (hp x 5250)/speed rpm, 5 hp @ 1185 rpm is about 22 lbf-ft, or foot-pounds if you run a torque wrench. Your gear reducer multiplies that torque by the ratio, approximately, and your force through the cables is the radius of the winch drum plus the height of the cable if not on the bottom wrap.

Guessing, Design B 150% x 22lbf-ft x 150:1 winch reducer is 4950lbf-ft to the winch drum. A 12" diameter drum on the bottom wrap is about 6" or 0.5ft radius, so your force is about 4950/0.5 = 9900 lb, distributed on your cables, or what mechanism you have, converting rotary motion to linear.

Take that force, and think of it at the end of a lever, maybe 3ft radius to your beam flange, now 30,000 lbf-ft moment distributed along the beam, and you are working up perhaps close to the yield strength of some steel sections. Part of that force is uplift on the far rail, so not all appears concentrated in one place. With this information, a real structural engineer (not me) will be able to figure out what portion is weak.

I took a guess that you monitor the water level before & after the grate, and use that to decide when to run the rake (High level difference in to out is some indication of plugging, or icing). That might help, but you will want to find out if more frequent cleaning will fix it.

If you buy that recording instrument or similar linked earlier, you can get an idea of under what conditions you see high load, and then decide what your options are. Leave it on the motor for a week or a month. Your local electrical contractor probably already has one, and then you could have them install upgraded electro-mechanical protection (shear pin) based on the readings.

If you can bend the steel with your current drive motor, you can see if you can run the rake with lower torque. For instance, stop the rake, manually clear what is causing higher than expected load. A current sensing relay on one motor lead can stop the motor, some modern overload relays have them included.

http://www2.schneider-electric.com/resources/sites/SCHNEIDER_ELECTRIC/content/live/FAQS/233000/FA233333/en_US/LT47%20-%20Instruction%20Sheet.pdf

thank you sooooo much man, your answers have been really good stuff.

I did some research and did find some good original drawings by the manufactor back in 98. See drawings bellow. I have also found some basic information about the rakes.

It is an Atlas polar model st8000. The lift capacity is 1250lbs and the whole thing weights in at 7385, as you can see in one of the drawings. The speed of the water is 2.85 feet per second. Theoretically it could go as deep as 34 ft bellow deck elevation.

Believe it or not, but the machine that needed repairing this past year was sent to be refurbished only 2 years ago. They did quite a few things:

1. Modifying the brushes on the trashrake head as well as the springs and replacing all of the fasteners with heavier diameters.
2. Redesign and fabrication of all of the trashrake boom guide rollers.
3. Replacement of the main boom outer 14” square tubes on both rakes.
4. Tweaking on the hydraulics in an attempt to smooth out operations to minimize the violent hammering that goes on when operating the rakes.
5. Installation of auto-lubrication systems.

so, all that said, with the new drawings if you could provide me with you final thoughts about what I could do to repair and test the machine prior to next year?

any final calculations would also be greatly appreciated.

21" 6 1/2'' is the distance to the top of the rake

It looks like all of the mechanical force that contributes to bending the raking unit support rails is developed by one or two hydraulic cylinders. As the rake is pulled up, that force tends to pull the raking unit towards the downstream trolley rail, creating uplift on the upstream flange, and downward force on the downstream flange.

The maximum force bending the rake unit trolley beams is the hydraulic pressure against the rake cylinder bore(s), pounds per square inch pressure against the area of the cylinder, less the cross-sectional area of the rod, in square inches (plus dead weight). The river hydraulic forces do not add to this force. So your worst case is a stalled rake. Your pump pressure relief can be adjusted to bring the force in line with the capacity of the structure, or you need to increase the strength of the structure to match the pump.

It is unclear from the drawings how to calculate the moment on the rails, as the attachment point of the rake cylinder end is not shown. We need to know the distance (radius) between the cylinder end and the racking trolley structure, to calculate the force, and the distance between the attachment point and the trolley wheel contact points.

The other thing to consider, what is your escalation response in the event of a stalled rake? It appears that the correct response needs to occur sooner, to ensure you are working within the limits of your equipment.

The hammering of the hydraulics is very concerning, for such a simple mechanism. I doubt whether you have determined the real root cause of that. Mechanical components are likely bending significantly while in operation; no amount of hydraulic circuit modification is going to fix that, just mask the real problem.

Sounds like loose pinions or pittman arms?

https://youtu.be/LiEl89IpxTk?t=193

Is this one like what Mr. Shippa has?

Maybe this is OP's real problem also, that there is operator that is attempting to reduce time operating the trash rake, and they wad it up on one end of the intake or the other, until the boom can't well lift it anymore, and something shears off?

Hey Rwilliams,

What grade steel would you use to build a new rail for the system?

I would stick with the original manufacturer specification (I assume Rail Steel ASTM-A-499), or what ever is reasonably available for that shape, as that is likely what it will get replaced with, for the next go-around.

If you use a special alloy or heat treatment, there may be other factors that were unofficially taken into account for the original design, such as elastic modulus or repetitive stress fatigue failure, that may appear.

The trick will be to make sure the load limiter (to avoid overstressing the equipment) settings are documented well for the next person, or the reinforcing added to strengthen the rail is obvious and done uniformly over the length of the rail.

Its hard to believe, but it was only A36 steel. Its obviously under design. I was looking into A572 Grade 50 for the new rail. thoughts?

I was initially going to say A36, but then did a little research. No one would be surprised to find actual rail steel there, which is probably even better adapted for the service, especially outdoor and wet, and even runs a little stronger than the A572 Grade 50.

The A36 moves a lot more before breaking, which you might think would be helpful, but it also can move towards more unstable positions, which can then add more strain than originally designed. Not sure how easy rail sections are to find in A36, if you were pumping these out 30 a year, the lower cost might be helpful to the OEM. (?)

A36 stronger than A572 grade 50? I was contemplating replacing the a36 for a572, with the min 50ksi yield in mind. The support beam is fine, the support plates also. Only the lower flange of the c channel seems to be taking most of the punishment. If I replaced the current channel with A572-50 grade then paint for rust and what not, not a good idea ?

No, the rail steel is stronger than either A36 of A572 Grade 50. I thought this was a rail section, it's a channel?

Maybe rail steel channel is better yet, there are shops that make different sections from rail steel, www.jssteel.com/rail-steel-vs-mild-steel, www.chs.com/railsteel.html and no need to paint.

I see now why it was A36 if a standard C section, you could try the intermediate alloy and see how it goes, or maybe check out Chicago Heights and see if they can make what you want. My mention of the A36 is that it is less brittle than the stronger metals, but I don't think that flexibility helps you in your application, probably hurts you for intermittent long slow overloads. You still need to look at limiting the torque input to the system so that your prime mover will trip before you start bending stuff.

More than likely your original system was woefully underdesigned for the current conditions at your inlet, and I suspect that only a properly engineered replacement will solve your problems.

Why is the GPM important? Suppose the expected "trash" was a 3" diameter, 3ft long branch weighing 50lbs, was traveling at a surface speed of 3 inches/sec. Now the surface speed is doubled due to an increase in the flow rate, how much additional momentum/impact/twisting force would the rake have to endure due to this increase?

Then consider what happens to the loading forces as the size and quantity doubles or quadruples; i.e., it's now a 12" diameter, 6ft long log weighing 400lbs traveling at 6 inches/sec. The forces have now increased dramatically, and the likelyhood of survivability decreases inversely. Bridges that have survived decades of storm-driven high water and debris are suddenly swept off their moorings because bigger, faster moving water/debris from a 100yr. storm comes along and hits them with forces that exceed their design safety factors.

Do it over and do it right, get a competent civil engineering firm to do a complete study of the river and design of the trash removal system. Although you may be able to repair/reuse some of the current system, the upstream conditions may have changed sufficiently to render the current design vulnerable to winding up in the turbine inlet, shutting you down completely.

It is true that engineering dwells in the details, and 9/10 times or even 10/10 times, certain assumptions are made based upon the initial project, but over the course of years, things change, without a competent engineering review being done.

a little abuse (away from the watchful eye) can go a long way, the motor current recorder can be a good diagnostic...

Other issues aside, the reason that the lower flange on the (wide-flange-rail-beam) has become (warped) is due to cummulative cyclic wear on it, causing the lower flange to fatigue, and further, to deform, as shown above. This would also contribute to the other supporting members slowly separating from each other.

When it wears out enough, you'll probably have to replace the entire assembly.

Also, good ''grill'' design includes ease of component replacement...

Clarifying, the above rail comment is in reference to the ''rail failed'' note in Post # 2...