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Expert Advice Needed

06/07/2017 12:08 PM

Hi Everyone, First of all, I have zero knowledge in engineering. However I would like to make a standard rocking chair to rock automatically. Can someone please give me some expert advice on how to do this, what materials will be needed, any reference or videos that I can read and watch to learn the basics? Thanks. Q

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#1

Re: Expert Advice Needed

06/07/2017 12:22 PM

Well you need to design it first....choose a design or look that you want...

click here...

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#6
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Re: Expert Advice Needed

06/07/2017 12:50 PM

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#17
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Re: Expert Advice Needed

06/07/2017 2:50 PM

thanks for the video. It's really helpful.

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#38
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Re: Expert Advice Needed

06/09/2017 6:39 AM

I like it ....but possibly the option of adding weights to the arm to accommodate a range of people.....and speed adjustment to find the natural frequency...

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#39
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Re: Expert Advice Needed

06/09/2017 7:48 AM

" ... but possibly the option of adding weights to the arm to accommodate a range of people.....and speed adjustment to find the natural frequency..." agreed - probably also need a more powerful motor and a longer arm for the counterweight.

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#58
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Re: Expert Advice Needed

06/10/2017 4:24 AM

I understand what you are saying, you are also echoing in my head my earlier thoughts.

BUT, on reflection, I feel that in reality, probably a lower frequency will be much more comfortable for a human. Maybe up to the natural frequency of a particular person....

The total weight of chair and "body" will probably dictate the speed for anyone in particular, lighter people maybe faster, and heavier, slower....just a thought.

Which is why a good wide ranging, variable speed is essential I feel......there are plenty of cheap powerful, simple circuits that can drive a DC motor (the wiper motor shown earlier?) with plenty of torque at a personal, pleasant frequency, for the "rider" concerned.....

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#60
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Re: Expert Advice Needed

06/10/2017 8:53 AM

Any person's "comfortable" frequency will be their natural resonant, or "harmonic" frequency. See #2.

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#62
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Re: Expert Advice Needed

06/11/2017 1:59 AM

I find it odd that several usually thoughtful people are supporting or otherwise going along with the idea that weight in and of itself has a large influence on the natural frequency of a rocking chair with or without cargo.

The resonant frequency of pendula are not dependent on magnitude of the mass at the end, so much as how far the center of mass is from the pivot.

Similarly, rocking chairs don't depend so much on the magnitude of the mass being supported so much as how far the center of mass is above the rocking surface relative to the quasi-pivot, i.e. the center of curvature of the rocker. As the height of the center of mass gets closer to the rocker center of curvature., the period gets longer.

If anything, a lighter rocker (with the same center of mass) will appear to rock slower as a similar amount of drag will yield greater damping than for a heavier rocker.

There are also plenty of rockers out there with high solid backs, so an empty rocker might not have a lower center of gravity than an occupied one.

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#64
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Re: Expert Advice Needed

06/11/2017 2:09 AM

Yes, the difference in natural frequency is fairly minor whether the chair is occupied or not. I've a few rockers here: a traditional wooden rocker and a bentwood, and there is not much difference when occupied or empty, which makes sense.

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#65
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Re: Expert Advice Needed

06/11/2017 8:48 AM

I do appreciate someone who defaults to impircism when doubts arise.

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#43
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Re: Expert Advice Needed

06/09/2017 11:19 AM

You could use a smaller mass if you accelerated and decelerated it at optimal points in its rotation. An intelligent controller could determine the chair's acceleration using an MEMS-type accelerometer directly or infer it from the motor current and shaft position, and vary the motor speed to bring the chair into resonance.

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#36
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Re: Expert Advice Needed

06/08/2017 1:24 PM

That motor, with possibly some speed reduction/adjustment, would to my mind be perfect as a start!

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#2

Re: Expert Advice Needed

06/07/2017 12:30 PM

There are a number of ways you can do this. One simple way to rock the chair is to place a motorised cam on one of the runners near the back. This has the disadvantage of causing wear marks where the cam contacts the floor, though this could be mitigated somewhat with a bearing on the cam lobe so that the tip rolls-on rather than scrapes against the floor. Some sort of wear-pad on the floor at that spot would help but the thing would look unsightly.

A much more elegant, non-contact solution and one a bit more complex is to have two counter-rotating eccentric masses that gently move the rocker's centre-of-gravity forward and backward at the chair+occupant's natural resonant frequency (the same physical principle underlying the working of a child's swing).

This device could be mounted underneath the seat, out-of-sight. Two brushless pancake motors, each driving its own mass in counter-sync with the other, their speed and position governed by an intelligent controller equipped with an accelerometer.

The controller would determine the resonant frequency by moving the masses at gradually increasing RPM whilst monitoring the chair's response with the accelerometer. It could also limit the chair's maximum excursion by varying the motor speed to actively counter the excursion, as necessary.

It could also automatically determine when the occupant is no longer in the chair by noting a sudden increase in the resonant frequency, and shut down automatically (even countering the rocking action first to stop the chair).

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#9
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Re: Expert Advice Needed

06/07/2017 1:38 PM

Thanks for the advice. what is motorised cam?

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#15
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Re: Expert Advice Needed

06/07/2017 2:44 PM
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#16
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Re: Expert Advice Needed

06/07/2017 2:49 PM

thanks.

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#30
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Re: Expert Advice Needed

06/08/2017 2:50 AM

"....It could also automatically determine when the occupant is no longer in the chair by noting a sudden increase in the resonant frequency...."

.

Hmm. Are you assuming the height of the center of mass increases when a person exits the chair?

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#3

Re: Expert Advice Needed

06/07/2017 12:43 PM

There are plenty of how-to books in your local home supply store,... check those out.

One thing to keep in mind is the ergonomics of the design. I worked at a large commercial furniture manufacturer, where comfort plays a great deal. And being a large volume manufacturer, they would make a number of proto-type to see their comfort and test their life.

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#4
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Re: Expert Advice Needed

06/07/2017 12:47 PM

From the sounds of it the OP is looking to retrofit an existing rocker.

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#5
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Re: Expert Advice Needed

06/07/2017 12:50 PM

I just finished reading it...

So,... I'll give this advice on a automatic rocker..... Move south and go green.

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#7
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Re: Expert Advice Needed

06/07/2017 1:03 PM

A breeze, and you are rocking. I like it!

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#8

Re: Expert Advice Needed

06/07/2017 1:12 PM

One name for a solution to your quest is a bateau. They come in a variety of sizes and can be used with or without your chair. Just add sufficient water.

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#10

Re: Expert Advice Needed

06/07/2017 2:05 PM

Is this for a play on stage involving spirits or ghosts?

Why? This has been done before, see S.E.'s post with the video of auto-rocker.

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#12
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Re: Expert Advice Needed

06/07/2017 2:29 PM

I'm hoping this is for a prop or a legitimately disabled person who can't rock a chair on their own because if it's for someone who has grown too fat and lazy to power their own chair.....

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#13
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Re: Expert Advice Needed

06/07/2017 2:37 PM

You are worrying too much

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#11

Re: Expert Advice Needed

06/07/2017 2:15 PM

Do you want to build the chair from scratch?

Or, buy a rocking chair and then motorise the motion?

Either way, I think I favour the type which have fixed feet/supports on the floor, and, the rails roll on built in rails, or, the chair is supported on swinging struts. I'm just a bit concerned here about the idea of falling asleep and "creeping" off the veranda/patio.

I think a lot of toys rely on hall effect sensors and an electromagnet to apply small pulses of energy as the motion crosses the mid point. You really don't need a lot of power as long as friction is kept to a minimum, and you can time the pulses correctly. Might be worth looking for the mechanism for one of those lucky waving cats.

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#14

Re: Expert Advice Needed

06/07/2017 2:43 PM

This video is basically what I want. Does anyone know where to get the similar motor? and what type of motor is that?

Thanks.

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#18
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Re: Expert Advice Needed

06/07/2017 3:01 PM

It doesn't need to be that complicated. Just attach the end of the cord to the edge of a rotating wheel, or just the end of a single rotating "spoke". You need to be a bit careful to make sure the cord doesn't tangle.

In particular you will need a "swivel" end attachment to stop the cord from twisting up.

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#20
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Re: Expert Advice Needed

06/07/2017 3:45 PM

Yes, we don't want to pull granny over now do we...

We don't want to "launch" granny either, if it tangles and "lets go".

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#19
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Re: Expert Advice Needed

06/07/2017 3:44 PM

Aren't electric motors outlawed in Amish communities?

JK. That is a motor with a geared down output and it looks like the mechanism is one from an oscillating fan. (eccentric cam probably, yes.)

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#50
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Re: Expert Advice Needed

06/09/2017 12:55 PM

My dad visited an Amish machine shop where all the machines were powered by compressed air. They bought the compressed air from the adjacent, non-Amish business who owned the compressor. Nothing in the shop proper was electrical.

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#51
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Re: Expert Advice Needed

06/09/2017 1:43 PM

I can see perpetual sillyness entering here:

Goldberg's automatic rocking chair:

  1. compressed air from bellows pump runs small air motor to rock the chair.
  2. Rocking chair actuates the bellows pump, thereby supplying needed compressed air to the air motor.
  3. Shifting of biomass with the rocking (it could happen) supposedly makes the cycle have over unity gain.
  4. Fat granny keeps on rockin' til the break of dawn.
  5. Cat slides tail under a runner, cat gets tail crushed by the immense pressure (that means hyuge in Rio Linda), Cat screeches, meows loudly, screams in agony while flailing all four legs in air, trying to purchase some traction.
  6. Fat Granny has a cardiac arrest by being so suddenly aroused from slumber.
  7. Rigor sets in after Granny passes, and the rocking stops. Perhaps sooner, since the cat shreds the bellows pump.
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#52
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Re: Expert Advice Needed

06/09/2017 2:39 PM

Party like it's 1699!

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#53
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Re: Expert Advice Needed

06/09/2017 2:54 PM

What, pray tell, happened in 1699? The start of Amish culture? It certainly is within the time frame of the great "argument" of the mountain and valley Anabaptists (be baptized once again).

They certainly do get a lot done without having that Mahindra tractor sitting around, or the John Deere either. I wonder if they drive buggies of their own making, or if they are Dodge, or John Deere.

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#54
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Re: Expert Advice Needed

06/09/2017 3:00 PM

You've given me an idea!

No animals (or grannies) were hurt in creating this design.

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#55
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Re: Expert Advice Needed

06/09/2017 3:41 PM

That's only going to go a few cat-lengths before cat gives up. Need to design a cunning rachet/cam mechanism to reset it after each cat-dash!

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#61
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Re: Expert Advice Needed

06/10/2017 10:17 AM

...meanwhile there is a chance of a catnap whilst waiting....

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#21
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Re: Expert Advice Needed

06/07/2017 4:40 PM

12v motors

Looks like about 30 rpm motor speed....

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#22
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Re: Expert Advice Needed

06/07/2017 4:52 PM

Is it quieter than that other video? I meant yeah, your video was really quiet.

I hope it is that good for reals.

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#23
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Re: Expert Advice Needed

06/07/2017 5:11 PM
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#25
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Re: Expert Advice Needed

06/07/2017 5:53 PM

This is great! It is exactly what I want. I thought no such product exist, have to build one. Much thanks.

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#27
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Re: Expert Advice Needed

06/07/2017 6:20 PM

You realise of course that this mechanism is designed to be connected between the movable part of a crib and its stationary frame. This isn't a problem where cribs are concerned because the mechanism is wholly contained within the single piece of furniture. A conventional rocking chair has no such frame and so you will need to connect one end of the mechanism to something that is not part of the chair.

Now, what if you want to move the chair? What if somebody else tries, not knowing it is connected? What if the chair is bumped? Kids clambering on the chair? Rocking chairs also tend to 'scoot' a bit on the floor whilst rocking. All of these will contribute to placing undue stress on the mechanism, possibly breaking it. Something to consider.

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#29
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Re: Expert Advice Needed

06/08/2017 1:51 AM

No problem for a glider....

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#37
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Re: Expert Advice Needed

06/08/2017 6:40 PM

Of course it will. It will rock cribs too; the OP isn't asking about rocking a glider, but about something that will rock a 'standard rocking chair.'

"However I would like to make a standard rocking chair to rock automatically."

Something along these lines, presumably?

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#40
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Re: Expert Advice Needed

06/09/2017 9:43 AM

How about a fairly large mass, in rotation on an eccentric radius arm that rotates under tuned speed mounted under the seat. The coupled motion can only work effectively when the mass is in the forward facing part of rotation, or the back facing part. No linkage whatsoever other than mount on the motor shaft. Use a short axial profile motor (pancake?), and have the mass basically fold back over the outer motor housing to save even more profile. I wish I had something on this computer I could use to sketch it properly.

You can see hard point mount pads, and presumably, the wiring needed could pass between chair seat and the up face of the motor, with the output shaft protruding below the down face. Lead weight on Pittman arm produces rocking motion, I suspect. (Sort of like an out of balance washing machine).

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#41
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Re: Expert Advice Needed

06/09/2017 9:52 AM

See comment #2

Two counter-rotating masses sync'd so that their CoM moves in a straight line fore and aft. A single eccentric mass would only wobble the chair (the same way your phone vibrator works).

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#44
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Re: Expert Advice Needed

06/09/2017 11:38 AM

"A much more elegant, non-contact solution and one a bit more complex is to have two counter-rotating eccentric masses that gently move the rocker's centre-of-gravity forward and backward at the chair+occupant's natural resonant frequency (the same physical principle underlying the working of a child's swing).

This device could be mounted underneath the seat, out-of-sight. Two brushless pancake motors, each driving its own mass in counter-sync with the other, their speed and position governed by an intelligent controller equipped with an accelerometer."

Could one eliminate the second motor by proper reverse 1:1 gearing of the two arms/weights? I suspect a simple planetary gear system could do this, although it might be hard to get N1/N3 = 1. Then have a single speed control knob, so the "sweet spot" can be tuned in.

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#45
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Re: Expert Advice Needed

06/09/2017 11:52 AM

Yes, you can. Most definitely. I have a box of them in fact, somewhere in my storage. A little larger than a silver dollar and about 3/4" thick. Bronze housing, steel gears. I hate to imagine what they originally cost.

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#46
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Re: Expert Advice Needed

06/09/2017 11:57 AM

Make it so, No.1!

Boston Gear, perhaps?

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#47
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Re: Expert Advice Needed

06/09/2017 12:03 PM

No idea who the mfr is. No markings.

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#48
In reply to #41

Re: Expert Advice Needed

06/09/2017 12:18 PM

If the shaft on which a single eccentric weight was mounted was horizontal rather than vertical there would be no lateral wobble of the chair, though the wheel would be more limited in size

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#49
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Re: Expert Advice Needed

06/09/2017 12:36 PM

See #42

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#42
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Re: Expert Advice Needed

06/09/2017 10:58 AM

A single rotating eccentric mass would work in the vertical plane, however, as in comment #6.

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#56
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Re: Expert Advice Needed

06/09/2017 5:41 PM

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#57
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Re: Expert Advice Needed

06/10/2017 12:57 AM

That would be a dangerous chair without the springs.

Have any idea what the matching bevels are on the supporting rocker tracks just after of the contact patch?

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#59
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Re: Expert Advice Needed

06/10/2017 8:49 AM

Maybe the upper is indented for stability?........I think I would add some guide plates to the outside of either side, attached to the lower frame for safety....I do like the idea of the rocking feet to be off the floor...that way the floor surface doesn't matter....

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#63
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06/11/2017 2:05 AM

I was referring to the radius of the rocking surface....looks short enough it might fall below someone's center of mass sitting in the chair. Probably wouldn't rock well enough with the springs attached if the rockers had a normal floor-sitting radius.

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#66
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06/11/2017 8:59 AM

The 'bevels' are from someone rocking back and off the rails on the right hand side. There is probably wear on the inside of the rocker to match.

I don't believe it went off on the left ever as there is no indication of wear.

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#67
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06/11/2017 9:04 AM

Very nice. Thank you.

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#24
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06/07/2017 5:16 PM

That mechanism is unnecessarily complicated, fragile, and will wear out soon. Watch it and notice the mechanism stalls when pulling back. The mechanism's period is faster than the rocker's causing it to fight against the rocker's motion. That design, moreover, is hazardous. It's a tripwire, basically. Not a good idea.

The ideal mechanism would not have a fixed mechanical linkage between the chair and something else not part of the chair. A small, spring-loaded, cam-driven foot, for example, mounted underneath and attached to the chair in some fashion. To reduce wear on the floor you could cover the bottom of the foot in thick felt.

The rotating eccentric-mass approach in post #6 is a an elegantly simple solution, but you would want to enclose the rotating element for safety reasons. For that approach I would include a user-operated speed control to fine-tune the rocking motion. This design is the approach I would use.

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#26
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Re: Expert Advice Needed

06/07/2017 6:01 PM

Thanks for your insight.

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#28

Re: Expert Advice Needed

06/07/2017 11:49 PM

How can you have zero knowledge in engineering, but know that a rocking chair can be made automatic?

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#31

Re: Expert Advice Needed

06/08/2017 3:29 AM

How to do it depends upon the drive mechanism to be used. The rest is an arrangement of linkages.

How about getting hold of some motorised and "playing"?

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#32

Re: Expert Advice Needed

06/08/2017 10:03 AM

A linear motor mounted between the rockers behind the seat, with the shaft pointing downwards to push down at regular intervals

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#33
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06/08/2017 10:29 AM

link failure, please fix. I like you linear motor design, but couldn't the same be done with a solenoid with a slow response time? I guess, then there would be a shock hazard by contacting the large capacitor required, LOL.

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#34
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06/08/2017 11:32 AM

Sorry, the link works for me, but it was of only one brand of motor anyway. I would prefer a linear motor to a solenoid because the distance extended can be controlled. You can start with a short push and proceed to a longer push as the rocking gets going.

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#35
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06/08/2017 12:07 PM

You're correct, I defer to your keen insight on this one!

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