Elastomer couplings are known to be “torsionally soft”

Torque is “rotational force”

Does this mean that elastomer couplings have less capacity for transmitting rotational energy ? Do they not transmit much energy?

(I tried to fix the wording in your question. If I changed it too much, you can roll the edit back.)

  • 2
    It means they're easy to twist.
    – Hot Licks
    Aug 4, 2018 at 12:25
  • 1
    I've not encountered the expression before, but my first thought would be it's a reference to the way you could transfer rotational energy to something like an uncooked egg by spinning it. Because the egg's "internally elastic", it has less inertia to start with - not so you'd notice, but in theory it would be "softer" to start rotating. Certainly you'd notice the difference if you span one raw and one cooked egg at the same time, then briefly pressed down on each to stop it rotating (because when you took your finger off, the raw one would carry on spinning a bit). That's my guess, anyway. Aug 4, 2018 at 12:33
  • @FumbleFingers That isn't what the term means. It's a descriptor in engineering primarily used for torsional dampers. Couplings between engines and shaft-driven loads have to be analyzed for shock loads and torque pulses, and the couplings have to be carefully selected to handle alignment issues while transmitting torque and surviving abuse. See page T-7 "Rubber (HTR and CR)" of Lovejoy catalog
    – Phil Sweet
    Aug 4, 2018 at 18:19
  • @FumbleFingers Sorry, found this in the review cue and didn't see any answers.
    – Phil Sweet
    Aug 4, 2018 at 18:31
  • @Phil: I'm no (bio-) engineer, but isn't the underlying mechanics of "torsional damping" exactly what goes on when you spin up a raw egg as described? Some of the kinetic energy you're trying to impart (to the egg, or through the damped coupling) temporarily gets stored as potential energy (by stretching/squeezing rubber, elastic polymers, or whatever). But unless / until those "internal energy stores" actually break, all of that potential energy will eventually be converted to rotational energy, so elastomer couplings do not have less capacity for transmitting rotational energy. Aug 5, 2018 at 12:52

2 Answers 2


Torsionally soft is a qualitative description of a mechanical device used mostly for shaft couplers. It indicates (to engineers) the sort of jobs the coupler is best suited for. Soft materials are more compliant. Torsionally soft devices are ones that accept some rotational displacement between parts. Different physical arrangements allow for couplers to be stiff with respect to some deflections and compliant in other axes. So one type may accept axial deflection but resist torsional deflection. A different design may be stiff axially and soft torsionally. So the term itself refers to the combination of material specs and design.

A torsionally soft device generally means that it permits a fair amount of "wind up" to occur between the shafts. Diesels have large torque pulsation and would cause a boat propeller to operate under constant high acceleration and deceleration. A carefully designed coupler and shaft combination can smooth that out. It absorbs some energy in the process, but can increase propulsive efficiency overall. Size and operating speed of the coupler determine the power delivery, so you can still deliver lots of power through soft couplers, you just need more of a softer coupler material to do it.



an example:

Highly Flexible Couplings voith, inc.

Highly Flexible Couplings, Torsionally Flexible Couplings, Vibration Dampers: ... shift resonance frequencies in a driveline out of the speed range. They also damp critical torsional vibration amplitudes and torque shocks.

Meaning they have the ability to transmit energy, but with less vibration torsion amplitudes and shock. Any loss of energy is allegedly made up by less wear and tear etc...

flex coupling

  • Inertial damping - that's the word I was fumbling for when I commented to the question. Aug 4, 2018 at 13:03
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    @FumbleFingers in SciFi, a necessity with FTL travel. (faster than light) lol
    – lbf
    Aug 4, 2018 at 13:07
  • haha - I see Wikipedia says If a ship is using 1 g constant acceleration, it will appear to get near the speed of light in about a year. But Star Wars movies would drag a bit if they had to get around like that! Aug 4, 2018 at 13:23

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