Dashpot Operation

I'd have thought the mechanisms were pretty similar to a linear damper ..like those found on a car..should be plenty of info on them? Vicosity and fluid shear stress sound like prime suspects to me.

Just a vane in a pot full of gloop innit? (the gloop may even be just air)

Del

Have you done much digging on the web?

I couldn't find much info straight off, but this description of a patent application gives some clues. There were pages & pages of google hits to wade through.

I/we use rotary dampers/dashpots to control the closure speed of radiation shutters that are closed (or the source is shielded) when power is removed - an electromagnetic clutch opens and the damper controls the drop rate - but since I'm and "electronics-type" guy, I confess I've just used them without looking into the workings. If I get a chance I'll ask some of the mech. guys - can probably get a supplier contact name who should be able to give the low-down.

The dampers we use are from Kinetrol. We use the continuous rotation type, with an adjustment knob which can control the drop time of a 10kg shutter over 250mm between about 5 to 6 seconds and virtually free-fall. The knob doesn't have a locking mechanism, but once it's set (it's quite stiff to turn) it stays set. Some of these have been working for over 10 years without adjustment.

FYI they use silicone fluid (polydimethyl siloxane) as a damping medium.

Edit - they also do a vane type which doesn't do continuous rotation, but is better suited to reciprocating action - may be better for damping out wheel wobble. I can't comment on them, as I've never used them.

Hi,

the silicone fluid is used for two reasons:

1. it is non-flammable

2. its viscosity is less temperature dependent than ordinary oil, but modern motor oils have additives that do similar: 10W-40 and 5W-40 and similar are much cheaper but as good with regard to viscosity/temperature.

A simple linear spring and damper can be made from a double action hydraulic cylinder with a small bypass-line to connect both sides and either a needle valve or changeable orifices to adjust the damping coefficient. A suitable spring connected in parallel - preferably around the cylinder will complete the device.

RHABE

Hi,

better stay with linear dampers, much easier.

We built a rotary damper designed for a robotic knee, to act as a spring at fast action and as a damper at slow action.

Spring is by entrapped air volume.

Damper is by sealing by narrow gap H - requiring toroidal shape of housing with H = 10 to 50 µm clearance and clever design near the axis.

Viscosity µ of air is near 18x10^-6 Ns/m2 and compression to pressure p is causing leakage V' measured in m³/s of V' = (dp/dL).B.H³/(12µL),

this is to be found in Physics books and named Hagen-Poiseuille formula,

dp/dL is the pressure gradient along the flow path , L is length of gap in direction of flow,

B is total circumference of gap measured perpendicular to L and H.

If there is considerable temperature change then adjust the viscosity proportional to the square root of absolute temperature. Gases go up in viscosity with temperature, fluids go down!

RHABE

I was informed rotary dampers use a series of pins through a circular plate in a viscous fluid. Damping can be varied by number of pins and viscosity. I am interested because I am building a modified saddle-chariot with a trailing offset fully castering third wheel which allows wheelchair users to drive a horse drawn vehicle on their own, in safety.

I want a steering system that will allow the horse to turn, which tends to be slow, but which will stop the tendency for a speed wobble to build up. The main cause of the speed wobble is that when I get nervous, ie pony bolts when I am driving it from a wheelchair, I lean forward removing weight from third wheel and reducing the effect of the caster angle.

I think setting it up so I can't lean forward will cure the problem, but if I could eliminate high speed wobble two ways I would feel happier.

Simon

Think about using electrophoretic material (or something similar, such as magnetostrictive material) to give you ability to adjust viscosity.

I have used Rotary Viscous Dampers from Enidine. The damping characteristic is determined by the viscosity of the fluid

Thanks for all the advice. Just to clear things up, I DO UNDERSTAND that viscosity is the primary property responsible for the damping. If you read my first post carefully, you will see that. However, I am somewhat unfamiliar with what sorts of mechanisms Kinetrol and Enidine use to harness the damping power of viscosity. Do they simply use a very small clearance of several micro-inches and have the viscous fluid in the gap? It seems to me that this by itself would not do the job.

Hi

I will see if I can find an old damaged one and bust it open to see what is inside. Actually I am now quite interested myself....

Brgds

Just by chance, I came across this polydimethylwhatsit (PDMS) stuff in this blog entry - http://cr4.globalspec.com/blogentry/6558/Contact-Lens-Monitors-Eyes-Dispenses-Medicine - I'd never heard of it before. If the stuff's gloopy enough to mix with a bit of silver & make 'wires' it must have a pretty huge viscosity - presumably the properties can be tailored somehow during manufacture. Don't think the gaps would have to be all that small.