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Speed Damping Mechanisms?

Speed Damping Mechanisms?

Speed Damping Mechanisms?

(OP)
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.

RE: Speed Damping Mechanisms?

A rack with a damped gear.

RE: Speed Damping Mechanisms?

A Rotary Damper connected to the rails

Heckler   americanflag
Sr. Mechanical Engineer
SWx 2007 SP 4.0 & Pro/E 2001
      o
  _`\(,_
(_)/ (_)

"Avoid the base hypocrisy of condemning in one man what you pass over in silence when committed by another." -- Theodore Roosevelt

RE: Speed Damping Mechanisms?

Rotary damper connected to monitor through a timing belt.

RE: Speed Damping Mechanisms?

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

RE: Speed Damping Mechanisms?

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.

RE: Speed Damping Mechanisms?

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

RE: Speed Damping Mechanisms?

(OP)
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?

RE: Speed Damping Mechanisms?

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
www.lumenflow.com

RE: Speed Damping Mechanisms?

(OP)
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.

RE: Speed Damping Mechanisms?

I think you are thinking backwards.

The defining function of a damper is T = f(n), 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.

RE: Speed Damping Mechanisms?

(OP)
"The defining function of a damper is T = f(n), 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(n) 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(n) to be be vertical at some relatively low rotation rate winky smile  

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.

RE: Speed Damping Mechanisms?

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

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Speed Damping Mechanisms?

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.

RE: Speed Damping Mechanisms?

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.

RE: Speed Damping Mechanisms?

couple the clutch to a series of rotary dampers!

RE: Speed Damping Mechanisms?

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

RE: Speed Damping Mechanisms?

(OP)
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.

RE: Speed Damping Mechanisms?

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
www.lumenflow.com

RE: Speed Damping Mechanisms?

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.

RE: Speed Damping Mechanisms?

With a copper bar (6 inch) in a 210 magnet rail,  I found the drag force to be 2.5 lb.in/sec.

Although there is fringing, most of the magnetic field is contained in the "U" of the magnet rail.  Things have to bee pretty close to be affected by the magnetic field.

RE: Speed Damping Mechanisms?

(OP)
Thanks tlee123. Regarding shock absorbers, to keep the original question simple I neglected to mention that we need the screen to come to a definite stop at the end.  This would trigger a position lock (either solenoid or electromagnetic) and then playback of a final 30 seconds of animation.  

As you point out, a fixed end stop is going to cause a harsh impact (even at our desired max of 0.5 ft/s).  We might have to rethink the design - perhaps a motor-positioned screen and a separate slide control.  Bumping a separate slider into an end stop wouldn't be an issue.  Further, the slide would become a target location for the screen (which can lag at its max velocity).  At the end, we could "ease" the screen into position.

Thanks to everyone for the suggestions.  This is our first experience in eng-tips - what a great online community.

RE: Speed Damping Mechanisms?

sreid, I was surprised to hear you got that much damping from the eddy current device.  Thanks for the info.

I looked into the magnet track from Parker/Trilogy.  It's pretty neat that you can buy long lengths or modular. However, the tracks are expensive.  A 48" long pc is $2500-$2700 (210 style) and futnuh would need about 4 of them.  Being non-contact, it would never wear out, so I imagine the cost would be justified only if it saw constant usage.



RE: Speed Damping Mechanisms?

How about just a lot of friction in the system?

KENAT, probably the least qualified checker you'll ever meet...

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