# Specification of linear speed dampers I'm looking at linear compression dampers and finding them most commonly specified only with a "Max Force" – e.g., 270 lbs.

However, since the damping mechanism is provided by hydraulic or pneumatic resistance, the damping force must be a function of the force applied. Here is a supplier that specifies the damping rate as the resulting "inches/second of motion given a 40lb load."

As best I can tell, neither of these figures completely describes the damping characteristics of a hydraulic compression damper. Nor do they describe the same characteristic.

Is there some standard model or specification of such dampers that can characterize the damping resistance or speed that will result from any force (within some design range)? (And do either of these sample specifications determine the model?)

## 2 Answers

I think: These ratings are showing the max speed allowed by the damper. Certainly for a specified load, there's a startup acceleration, and further the manufacturers are assuming the load is applied nowhere near an impulse/delta function. Every system has a resonant frequency, meaning that if the load were applied in a very short pulse (or even a short rise time) the damper would transmit the shock rather than absorb or respond with resistive force.
The intended applications don't involve impulse forces, which is why (I'm guessing) the PSD is not of interest.

While automotive shock absorbers are not exactly the same as these dampers, they can be illustrative. They're designed to limit the amount of energy transferred from the wheels to the chassis, and more important,to spread impulse input energy over a much longer output period, thus smoothing the ride. But if you go over a bump at just the right speed, the shock absorber will 'resonate' and collapse without transmitting any energy, releasing the energy back to the tire/road after the bump.

• Right, it would be nice to know the impulse response range too! I was actually just wondering about resistive force or travel rate as a function of a constant force. (E.g., if we knew they can provide resistance up to 270lbs, and that at 40lbs they move at 1in/sec, what is their motion rate under other constant loads as we vary the load from 10lb-270lb?) – feetwet Jun 28 '17 at 13:53

The OP's second link, gives damping rates albeit in a most confusing way. If you work it out, for example, the first damper has 1.4 in/sec as the damping and references 40 lbf which means that the damper applies 40lbf when a velocity of 1.4 in/sec is applied. In this case: $$C_{example} = \frac{40(lbf)}{1.4 \frac{in}{sec}} = 343(\frac{lbf\cdot sec}{ft})$$

Which you multiply by the velocity to get the force:

$$F_{damping}(lbf) = V (\frac{ft}{s})*C(\frac{lbf\cdot sec}{ft})$$

• So does this map back into a damping model that can relate damping force to velocity? – feetwet Jun 28 '17 at 16:36
• Yes, see my edit above – DLS3141 Jun 28 '17 at 19:11
• Well the units on your equation check out, but is the damping relationship between velocity and force in fact linearly proportional? And if so, for which damping mechanisms does that relationship hold? E.g., I might suspect such a relationship to only apply (if anywhere) to a simple piston damper using an ideal fluid. – feetwet Jun 28 '17 at 19:24
• That will depend on the properties of the specific damper. It's likely going to be mostly linear over some range of velocities, but the boundaries of that range and the damper's behavior at the extremes are going to be design dependent and deviate from the simple linear model the manufacturer provides. There's also likely to be hysteresis and temperature dependence as well. If you need more details or a more precise model, you need to ask the manufacturer or do testing on your own to determine what those are. You should also expect some amount of part-to-part variation on the damping as well – DLS3141 Jun 28 '17 at 19:33