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DIY Shock: Rubber vs Hydraulic Damping + Information Help
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(Mechanical)
(OP)
21 Jul 16 14:34I'm helping a friend design a coilover shock for a wheelchair for his nephew. He suggested using an o-ring or seal friction damper, but after not finding much information to aid in design I wondered if using hydraulics would be viable, but I couldn't find much information on that either. I'm wondering two things: Which would be cheaper in the long term, and where can I find information about design details? He works in a machine shop and will be fabricating everything himself.The way I see it, a rubber damper might suffer from wear and need friction elements replaced fairly frequently (also giving a declining performance), whereas the hydraulic damper might be (slightly?) more expensive to produce but last longer between maintenance schedules (and give more uniform performance over time).Both designs have stumbling blocks. I can't find any guidelines on gland dimensions, seal material, or required surface finish for any friction dampers. And for the hydraulic shock how is oil inserted and topped up? I imagine some kind of valve but again, I couldn't find any information on this either.Preliminary designs are below for reference.Elastomeric:Hydraulic:
Replies continue below
(Mechanical)
21 Jul 16 14:56In the top one, are you saying the damping will be from the friction of the rubber O-rings against teh internal walls of the cylinder? Does seem like you may have wear issues. It will also be acting as a pneumatic spring won't it - unless you have a relief valve I don't see.If you're going to use rubber, why not use the rubber as both the spring & the damper? As I recall the DeHaviland Mosquito used a stack of O-rings as it's suspension. Googling 'rubber spring suspension' got me some hits that explained the idea.
Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)(Automotive)
21 Jul 16 15:38For hydraulics you just unscrew the top and pour oil in. If you want get fancy put a fill plug or Schrader valve in the bottom. Look at mountain bikes for ideas, personally I think you'd be nuts to go with rubber.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
(Mechanical)
21 Jul 16 15:53ChrisDanger,A lot of commercial and military anti-vibration mounts are stuffed with wire mesh to provide damping.How badly do you need damping on this thing, or an automotive style suspension? Wheelchairs do not go very fast. If I were designing one, I would go for sturdy, light weight, stable and cheap. I don't see a suspension helping with this.
--
JHG
(Mechanical)
21 Jul 16 16:06In the first drawing, you would probably get some leakage past the triple o-rings, thus a pneumatic damper. If you deliberately created a leak path for the air (via an orifice, or cut gaps in the o-rings) you could actually control/tune the amount of damping, just like you could by sizing the orifice(s) in the hydraulic fluid version's piston.
(Mechanical)
(OP)
21 Jul 16 16:16Hi KENAT. Thanks, yeah, I should've added some kind of pneumatic relief to the design.Greg, I was thinking of just filling with oil and screwing in the plug but wasn't sure if an excess pressure would damage the seal if it was overfilled. I'll look at putting a valve in, and at mountain bike shocks for ideas. Thanks. (PS Thought I recognised your name from f1t. Searched and first result was a thread on dampers/shocksdrawoh, I've used a wheelchair without suspension and it's not fun. I wouldn't want to subject the little guy to any more discomfort than is possible.Thanks for your help guys.EDIT: btrueblood, yeah, unintentional design flaw there. Damping was supposed to be from friction between the triple rings and inner cylinder surface.
(Mechanical)
21 Jul 16 17:02Proper Friction Snubbers use friction material that is basically the same as brake pad material.I think rubber o-rings would not last very long.
(Mechanical)
(OP)
21 Jul 16 17:19Hi MintJulep. Thanks for the info. I'll look into this option too.
(Mechanical)
21 Jul 16 17:20You should be able to find a coil over damper to suit your application at far lower cost and fewer tears than DIY. Mountain bike sounds like a great starting place.
je suis charlie
(Industrial)
21 Jul 16 17:23My dayjob is currently in medical equipment, so I deal with a lot of these parts and components. Presuming that your friend's wheelchair is electric, have you absolutely verified that there is no suspension currently in the chassis? Many (most) chairs do have some suspension. Get ahold of the manual, and barring that, talk to the technician at the dealer. There are also some really good "flotation" cushions available to reduce pressure concentration, ROHO being one of the better ones.
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
Want more information on Rubber Damper? Feel free to contact us.
(Mechanical)
(OP)
21 Jul 16 17:51Hi gruntguru. Yeah, we're looking at maybe producing a few of these for the other kids at the school, so it may not be a 1-off. My friend is keen to make everything himself for that custom look. He only has to pay for material, and tears we can handle. Hopefully we get it more or less right first time, although clearly at this stage we don't really know what we're doing. I have plenty of free time though, and am eager to apply some of my engineering knowledge.
Hi ornerynorsk. We're fabricating this chair (manual, not electric) totally from scratch. Thanks for the advice though. Will pass the ROHO recommendation on.
(Industrial)
21 Jul 16 18:17OK, got it. Mountain bike shocks as a previous poster mentioned are well worth consideration, as are non-locking gas springs.This http://softwheel.technology/theacrobat/ may also be of interest.
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
(Mechanical)
(OP)
21 Jul 16 18:51Thanks. Yeah, I saw a video of those. Very interesting concept. A bit like the below, but using a more conventional method.
(Mechanical)
21 Jul 16 19:30You can buy an off-the-shelf MTB shock for less than $50. Considering what's required to make a shock that actually works (high-end materials, hardened shaft with very fine chroming, intricately machined parts with fine surface finishes, etc) I doubt your buddy will be able to beat that, even only paying for materials.
A shock designed for a mountain bike, if used on a wheelchair, is going to last probably until the end of time. They are low maintenance as it is, and a shock on a mountain bike takes a LOT of abuse.
If you want to take it on as a design exercise then that's fun, but for cost and performance you won't be able to beat something off the shelf.
(Mechanical)
21 Jul 16 20:09 http://u.cubeupload.com/ChrisDanger/Hydraulic.pngWill this damper be mounted shaft ~ vertical?When the damper compresses, the volume of the shaft reduces the space within the shock.If the shock is full of oil the damper won't be able to compress at all.Real shocks provide a volume of gas to compress so the displaced oil has some place to go when the shock is compressed/shortened.A few include a volume of gas that was intended to mix with the oil and form an emulsion, and the valving in the shock was designed to perform with the emulsion, not "pure" oil.British manufacturer Girling offered an emulsion shock for off road motor cycles.
(Mechanical)
(OP)
22 Jul 16 10:20Riiiiiight. I did see the gas-filled chamber but I thought this was to regulate oil pressure to prevent cavitation which I figured wouldn't be a problem in this application. So that shock will not work. That's quite a serious oversight, thanks!
I'm starting to lean towards buying one. They're not so cheap or available here in South Africa but I'll see what I can do.
(Mechanical)
22 Jul 16 14:11You could make a damper with a "counterbalancing " extended shaft extension for ~ constant internal volume, but that brings on twice as much sealing trouble and complicates manufacturing.And some space is still needed for thermal expansion of the oil.
(Industrial)
22 Jul 16 15:38Anything on Bid or Buy? Any local bike shops? Ebay (US) has scads of them, most shipping from China at fairly reasonable prices. Just a thought. Best wishes on the project!
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
(Mechanical)
(OP)
23 Jul 16 11:36Interesting, thanks Tmoose.
Hi ornerynorsk. Nothing on Bid or Buy. Found some places in China but trying to get prices and see if they'll do small volumes.
(Automotive)
23 Jul 16 12:03If the shock is upright then you just need to leave room for stroke * rod CSA for the oil that is displaced. That is common practice with monoshocks. I see no point in gas pressurisation for any likely usage profile.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
(Mechanical)
(OP)
24 Jul 16 07:13Thanks Greg. Good to know.
(Automotive)
24 Jul 16 09:34Bear in mind that all the energy burnt in the shock absorbers has to be supplied by your power source. Human powered vehicles are very sensitive to this.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
(Petroleum)
24 Jul 16 12:50Have you come across these guys http://www.suspa.com/uk/sectors/medical-laboratory... Maybe take a peek at a truck drivers chair to see what they use.Also these guys. They do custom units. http://timbren.com/aeon-springs/ Depends on what your deflection and load requirements are.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
(Structural)
25 Jul 16 14:05I agree with Greg. Bump energy is essentially absorbed by a reaction provided by the user. Springs reflect more force reaction to a given bumps than good hydraulic dampers. Has this been engineered yet?
I've got a write up on this from my Ariel Atom post that I will add.
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Posted 07 November - 15:17
Greg,In response to your query re horsepower and life the following are extracts from the SAE paper by W. Flower (Lord Corporation) who developed this type of mount.]Rubber Hysteresis Friction (RHF) DAMPINGAn enlarged view of the interface between the elastomeric substrate and the slider indenter is shown in figure 19. The slider indents and travels along the elastomeric layer. Ahead of the slider, a small "bow wave" is built up; and behind the slider, a "trough" exists which gradually recovers from the slider indentation. The magnitude of the bow wave and trough depend on the indentation, velocity of sliding and various elastomer properties. A non-symmetrical, non-uniform pressure field is build up about the slider/elastomer contact patch area opposing slider motion. This results in a motion retarding force composed of frictional and hystereticcomponents. This is the force which provides the damping in the RHF damped mount system.The situation is more complex for the case of multiple, closely spaced sliders, since the recovery properties of the elastomer become critical. If elastomer recovery is too slow, following sliders do not indent the elastomer to the degree that the initial slider does, and the damping from friction is lessened. However, if elastomer recovery is too rapid, the bulk hysteresis loss in the elastomer is reduced and the damping effect from hysteresis is reduced. Both friction and hysteresis damping effects are important.TEMPERATURE SENSITIVITY OF RHF DAMPINGThe temperature sensitivity of elastomer damping is well known]. Tests were conducted on a RHF damper to determine the stiffening influence of low temperatures. Figure 20 shows the ratio of loop area at -40°C to loop area at 20°C obtained at a common input amplitude and frequency. For the first cycle of excitation, at the beginning of the test sequence, the loop area at -40°C is almost three times the loop area measured at room temperature. After accumulating 100 cycles, the loop area ratio drops to about 2.7, the cycling is then discontinued until the next test time. Following one hour from time 0, the loop area ratio for the first cycle is about 2.8, and 2.5 for the 100th cycle. These ratios are approximately constant with accumulated time. Figure 21 shows the same loop area ratio information, but obtained at -20°C relative to 20°C. This data does not include the heating effect of continued cycling and the resultant reduction in loop area ratio.FATIGUE OF RFH DAMPER MOUNTSingle axis fatigue tests were conducted using the dynamic load and test frequency versus cycles spectrum shown in figure 22 which is typical of the vertical loads seen on a medium duty truck engine mount. The dynamic loads were superimposed on a static load of 408 kgm to simulate engine weight per mount. Figure 23 indicates how the room temperature loop area is retained as fatigue cycles are accumulated. The retention of loop area with cycle accumulation suggests consistent dynamic performance is feasible.For the RHF damped mount the hysteresis loop curve in figure 13 shows a nonlinear curve suggestive of a mount with supplemental snubbing. Actually the non-linear effect is due to the onset of RHF damping. Up to about ±1.0 mm, the damping function is disconnected or decoupled from the simple rubber mount. The loop area is only 0.09 kN-mm/cycle, but at inputs above ± 1.0 mm, damping from RHF components is added to damping from the simple rubber mount. This is shown more clearly in figure 14 which was taken at ±5.0 mm and 1 Hz. The shaded area indicates the contribution of the RHF damper. The loop area becomes 10.12 kN-mm/cycle.SUMMARYRHF damped mounts are shown to provide damping performance somewhat different than similarly sized fluid mounts. They are primarily amplitude sensitive and not strongly frequency sensitive. They will not provide a minimum stiffness at certain frequencies and amplitudes; but neither do they develop significant stiffness increase at high frequency and low amplitudes. Both types of mounts offer amplitude decoupling for enhanced low amplitude isolation.I hope this is of help.
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