Speed Damping Mechanisms?

[futnuh]

I'm working on a museum exhibit where a 65" plasma monitor (150lb) is manually moved back and forth along a 15' linear rail. We're looking for a passive (non-electronic) means of limiting the speed to thwart kids flinging the screen from one end to the other. We want the screen to move freely at slow speeds but max out at ~0.5 ft/s. I may be wrong but the long 15' run and seems to preclude using a cylinder. Are there stock solutions to this problem? As always, any advice is much appreciated.

 

[TheTick]

A rack with a damped gear.

 

[Heckler]

A Rotary Damper connected to the rails



Heckler
Sr. Mechanical Engineer
SWx 2007 SP 4.0 & Pro/E 2001
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"Avoid the base hypocrisy of condemning in one man what you pass over in silence when committed by another." -- Theodore Roosevelt

 

[MintJulep]

Rotary damper connected to monitor through a timing belt.

 

[fcsuper]

The rotary dampen is prolly the best option, but I thought I'd just through out some other ideas:

Hydraulic system
Springs
Leadscrew
Insertia reel no locking reactor (ala, common seat belt)
Attach a hundred helium balloons to the top
Security Guard that wages their finger at the kids




Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA
sw.fcsuper.com
Co-moderator of Solidworks Yahoo! Group

 

[btrueblood]

Go down to the local supermarket, and roll some shopping carts around the aisles at high speed. Or take my 9-yr. old to help you. Inevitably, the cart chosen will have a misaligned wheel, or loose bearing, and wobbles & skids at any speed above a geriatric crawl. Take the cart home, cut off the wheel, and use it for your rail trolley.

 

[Compositepro]

A centrifugal clutch, as you might find on a chain saw, could be used as a brake. It would provide no braking until the the speed exceeded the set speed

 

[futnuh]

Thanks so much for the quick responses. A rotary damper seems like the ideal solution to solve the "flinging problem", the case when a temporary force is applied horizontally to the screen. Even if the initial impulse/shove is large, the velocity can be made to damp relatively quickly.

My concern is that the torque vs. RPM curves for the few rotary dampers I've looked become linear rather than asymptotic (or even quadratic). That is, rather than maxing out the RPM with increased applied force, you can always get them spinning faster. This is a worry where forces range from gentle (an older person pushing gently to move the screen) to extreme (a pack of rambunctious hellions throwing all of their weight into it).

Take, for example, this product from Enidine,

http://www.enidine.com/RVDmain.html#specs

http://www.enidine.com/pdffiles/RVD-Innovation.pdf

(curves)
They have relatively-low breakaway torque (3-6 in-lbs) which is nice - means that (ignoring inertia and rail friction), the screen can move with a gentle push. However, even for the most viscous option, 40 in-lbs gives you 500 RPM.

Or am I misunderstanding these engineering curves?

 

[lumenharold]

It might take quite a few to be effective considering the 150lb load but what about magnetic damping? A bunch of N40 magnets with a backer plate passing over an aluminum plate makes for a very simple mechanism. At slow speeds there is little drag but the faster the cart travels the higher the drag. Only moving parts are the wheels.

Harold
SW2007 SP2.2 OPW2007 SP0.0

http://www.lumenflow.com

 

[futnuh]

Re the centrifugal clutch, any sense about minimum RPMs required. I can, of course, gear up - but the main rack drive won't exceed 50 RPM. And we'd like the whole unit to be essentially noiseless.

 

[MintJulep]

I think you are thinking backwards.

The defining function of a damper is T = f, where T is torque and n is shaft rpm.

The faster you try to spin the shaft, the more torque the damper resists with.

Something like this:

http://www.efdyn.com/uploads/379.pdf

would seem appropriate.

 

[futnuh]

"The defining function of a damper is T = f, where T is torque and n is shaft rpm. The faster you try to spin the shaft, the more torque the damper resists with."

Yes, I understand this ... but (and I might be wrong) the details of the function f matter. If torque is linearly proportional to rpm, the kids might still be able to get the screen moving at a fair lateral speed. Ideally we'd like f to be be vertical at some relatively low rotation rate ;-)

I guess we just spec the damper based on the maximum reasonable force that a person can exert, figure out the corresponding rpm, convert this to a lateral speed (depends on details of the rack and pinion system, and then get damping end stops capable of decelerating the screen.

 

[electricpete]

The centrifugal clutch mentioned is a non-linear option that kicks in once you exceed a threshhold.

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[MintJulep]

But what does the centrifugal clutch engage against?

I can envision flinging the panel against the clutch engagement. If the clutch is coupled to "ground" on the other side I think that would be nearly as bad as flinging against a hard stop.

Don't forget that you can add multiple dampers in parallel and the torques would be additive.

 

[amorrison]

Suggest an energy dissipating fan blade like they have on some exercise bicycles (blades paddlewheel fashion) or a regular(room/desk) circulation fan if previous option not available or buildable.
The power "loss" varies with the cube of the rpm.
No working parts or electrical power required.
Install a power input "pulley" on the same shaft as the fan.

 

[ivymike]

couple the clutch to a series of rotary dampers!

 

[sreid]

I want to elaborate on Lumenharold's magnetic eddy current brake idea. Linear Motor Magnet Rails are an easily mountable magnetic field source. An aluminum blade is put where the linear motor forcer would go. The amount of linear drag vs. velocity that can be generated is truly impressive. As a test, purchase a standard 9.6 inch length of a Trilogy 310 magnet rail and experiment. The force constant will be proportional to the length of the magnet rail. You would attach the magnet rail to the screen and the aluminum blade would be fixed.

http://www.parkermotion.com/products/Linear_Servo_Motors__7011__30_32_80_567_29.html

 

[futnuh]

Thanks sreid and lumenharold, I like the magnetic damping option as unlike the rotary dampers, the breakaway force/torque is 0. Our one concern with strong magnetics is their propensity for erasing credit cards and computer disks. Back in my undergrad days, we had MRI magnets that would wipe a bank card just by stepping into the room. Do you have any sense of the field leakage of say the Trilogy 310 magnet rail? We will have a computer riding along with the screen on the rail - its hard-drive is probably sensitive to the magnetic field.

 

[lumenharold]

I'm not familiar with the Trliogy product. All the applications I've been involved with have been custom assemblies. We located our magnets as far away from sensitve electronics and memory devices as possible and took advantage of the magnet backer plate as an isolator.


Harold
SW2007 SP2.2 OPW2007 SP0.0

http://www.lumenflow.com

 

[tlee123]

First, an eddy current damper (the "magnetic damping") can be useful but is not appropriate here. This frictionless damping will be too low unless you use huge neodymium magnets with thick copper or aluminum plates.

Second, a rack driving a rotary damper is a great idea. I think you won't want to have the damping set too high (as you mentioned) because you'll need kids and older people to be able to move it. Even with low damping, it can absorb energy and slow the panel after a hard shove. However, I understand your worry that a sustained push can get it going at a good rate.

Third, I think the problem here is not just the velocity, it's mostly the harsh impact at the stops. Why not use some springs or shock absorbers at the end of travel? Look into Ace Controls or Enidine.