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# ROTARY STIFFNESS. What does it mean?

## ROTARY STIFFNESS. What does it mean?

(OP)
Hi Guys,

I have a Rextroth hydraulic motor -AA6V-M55HD2--/63W—VSD510---A--
What does it mean by rotary stiffness = 5670Nm/rad?

R.Efendy

### RE: ROTARY STIFFNESS. What does it mean?

I'll take a punt and suggest that that is the effective stiffness in series with the shaft. It'll be related to the compressibility of the oil and other elastic effects.

Cheers

Greg Locock

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### RE: ROTARY STIFFNESS. What does it mean?

(OP)
can i get rated torque from there?
For example,

R.Efendy

### RE: ROTARY STIFFNESS. What does it mean?

I would suggest it is what is says. The resistance against rotation. Such values are used in torsional vibration calculations. are you sure it is 5670 Nm/rad and not kNm/rad because i think the mentioned value is quite low.

### RE: ROTARY STIFFNESS. What does it mean?

I don't do metric that much but here is its counterpart in the english system. aThink of spring stiffness in lbs/in; now think of torsion thickness in ft-lbs/radian.

### RE: ROTARY STIFFNESS. What does it mean?

Oops, substitute "torsion thickness" for "torsion stiffness".

### RE: ROTARY STIFFNESS. What does it mean?

BEMPE16524,

I cannot be bothered to look up your hydraulic motor. Is it a torque device or an actuator? The torsional stiffness may be a measure of how accurately it positions something. Whatever is attached to your motor has a moment of inertia. This, coupled with your motor, is a spring and mass system with a natural frequency. Maybe this matters to you.

Rated (maximum allowable) torque is based on how your system will fail. Rotary stiffness does not predict something breaking. It could predict something not being rigidly positioned enough. It could predict something amplifying applied vibrations.

--
JHG

### RE: ROTARY STIFFNESS. What does it mean?

(OP)
drawoh,

we use it to drive a centrifugal pump. the whole system is submerged in subsea. actually i made a mistake, the rotary stifness is 32000Nm/rad max.
I still wonder how can this motor has such a very low torque (mentioned as 349Nm). is that value meant during high speed only?

R.Efendy

### RE: ROTARY STIFFNESS. What does it mean?

BEMPE16524,

I have no frame of reference to judge whether or not 349N.m is low torque. Sometimes, it helps to analyze a shaft approximately the diameter and length of whatever is in that motor. Work out the torque that generates yield stress, and work out torsional rigidity. They may turn out to be close to those of your motor. Gut feelings about things should be based on real experience and/or analysis. Don't go guessing.

--
JHG

### RE: ROTARY STIFFNESS. What does it mean?

Rotary stiffness is the torsional spring rate of the shaft, combined with any other elasticity in the system (as Greg already mentioned).

Stiffness and tolerable loading are not the same thing. If the motor is rated for 350Nm of torque, that is the max force it can apply, and also (but not necessarily) the max force it can react against while holding position.

The stiffness value just gives you a spring rate, so that you can calculate how much the pump will deflect in torsion under a given load.

This is exactly the same as loading a piece of steel. Young's Modulus for A36 steel is 29,000,000 psi. This does not mean a piece of A36 can withstand 29,000,000 psi of stress. Apples and oranges.

### RE: ROTARY STIFFNESS. What does it mean?

BEMPE16524,

I have one more thought. Is this an underwater centrifugal pump motor, or is it a motor for anyone who may need a hydraulic motor? If it is the latter, the manufacturer will tell you everything anyone might want to know about it. The stiffness may matter if the motor is an actuator accelerating and decelerating heavy objects. For a pump, your impeller should be light, and the stiffness should not matter.

--
JHG

### RE: ROTARY STIFFNESS. What does it mean?

(OP)
drawoh,

It is for underwater use together with a Fybroc 1500 (4x4x10) centrifugal pump.
I'm on the same frequency with you regarding the light impeller. Only my concern is once it is activated in water, isn't the initial load to rotate the impeller is quite high? (i don't have any data).
and one more problem, this is a on/off action only on the hydraulic motor with no pressure/flow control.

See my attachment for the GA.

R.Efendy

### RE: ROTARY STIFFNESS. What does it mean?

The torque rating is not speed dependant. It is pressure and displacement dependant.
Why do you think the torque is low for the motor? The chart shows torque at maximum displacement at 400 bar.
T = D*p/20π. T(Nm) D(cc/r) p(bar)
Ted

### RE: ROTARY STIFFNESS. What does it mean?

(OP)
base on the equation, T is increase if D or p or both are increase and vice versa. correct me if i'm wrong.

R.Efendy

### RE: ROTARY STIFFNESS. What does it mean?

You are correct.

Ted

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