How to calculate stopping force and stopping torque 1

[Startto]

I have a bundle of steel bars with each bar measuring length about 1.5m. Total weight of bundle is 650kg.If I fasten the steel bar bundle to a hydraulic motor in centre position of the bundle(.75m to each end) and rotate it with 800 Nm of torque.How to calculate the stopping force and stopping torque produced on sudden shutting down of the motor.

During sudden shutdown a sudden pressure hike upto 85 bar is witness in return flow.
where as,inlet is supplied with 22.9 L/min flow with 75 bar pressure.

 

[kiwitom235]

do you use a crossport relief valve, and if so what is it set to?

 

[Startto]

@kiwitom235
Hi Friend,
Yes we use crossport valve, but I don't know what's the maximum value. I hope it should between 180 to 250 bar

 

[rothers]

torque = inertia x angular deceleration

 

[dvd]

You could gain some insights into this by studying literature from shock absorber manufacturers. Kinetic energy and propelling force may need to be considered. Is this bundle on a roller bed conveyor (which may add kinetic energy)? If the rollers are stopped too quickly, the bundle will slide on the rollers.

 

[hydtools]

Sounds like the bundle is being turned like a propeller.

Ted

 

[SnTMan]

rothers, you're saying it works the same for acceleration or deceleration? Wow, who knew

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[chicopee]

To lift a bundle of steel bar at its center is a bad idea. Safer would be to use a sling and pick that bundle at two points along the length of the bundle.

 

[hydtools]

How sudden is the stopping?
Angular impulse = change in angular momentum
Torque x time to stop = final momentum - initial momentum. Final momentum = 0 where angular velocity is 0.

Ted

 

[hydtools]

If 800 Nm is produced by 75 bar then you know that 85 bar will produce 907 Nm. The net peaak retarding torque will be 907 - 800 = 107 Nm.

Ted

 

[hydtools]

The result of applying the equations is interesting. The torque to stop the spinning is independent of the moment of inertia.
Torque to stop = T2
Initial running torque = T1
Initial running time = t1
Stopping time = delta t

T2 = - T1 * t1/ delta t. The minus sign means T2 is in the direction opposite T1.

Ted

 

[kiwitom235]

@hydtools i am not sure where you are getting your equations from? what you are saying doesn't seem to make sense.

By newtons second law there is no defined 'torque to stop' unless you define the time to stop and moment of inertial of the rotating object.

 

[handleman]

I think what hydtools means is that you don't necessarily need to know the moment of inertia if you know torques and times because it affects both the starting and the stopping in the same way, so it ends up on both sides of the equation and it drops out.

 

[MintJulep]

As succinctly stated by rothers:

torque = inertia x angular deceleration

The only way inertia "drops out" is if there is no angular acceleration.

 

[hydtools]

I looked at the problem in two time frames. The first being time from 0 to t1, spinup time. The second being time from t1 to t2, stopping time.

Ted

 

[MintJulep]

t1 --> unknown
Δt --> unknown
T1 --> unknown. We are told that it was 800 Nm when the motor was operating. But the motor has been shut down.

T2 = product of three unknowns.

 

[hydtools]

Measure t1 while under the influence of T1 = 800 Nm
Measure delta t from time shutdown valve is closed and load stops.

Something has to be measured or known. Otherwise all you have are unknowns. I assume the OP asks how to find unknown stopping torque when you know starting torque, run time and maybe stop time. Or measure various deltaP across the motor and calculate torque if you know the motor displacement. OP does not mention motor displacement, so that is an unknown. How is 800 Nm known?

Ted

 

[BrianE22]

Startto -

Looks like you're trying to use the pressure readings across the pump to estimate how much torque is being applied. First, you need DeltaP (not just the return line pressure). Then you need to work from your motor to the point you want to know the torque. There will be rotational inertias and rotational stiffness that will have to be considered. Plus, depending on how sudden the braking is you may need electronics to monitor the deltaP and not just a pressure gauge.

 

[hydtools]

Motor torque, Nm = deltaP(bar) x displ(cc/rev) x mech.eff./62.83

Ted

 

[kiwitom235]

in my opinion everyone is over complicating this.

My thoughts...

assume the delta P is the pressure at the return line, as The motor inlet pressure will be near zero very quickly as the supply has been shut down.... hence zero supply pressure.

You need to know inertia of the system and angular velocity. it is easy enough to work out the torque applied as you know the motor outlet pressure (just make the assumption it is constant for simplicity sake).

from there you can work out the time to reach angular velocity of zero.

It is silly to look at start up torque and time to start up, you need to look at the system conditions at the point of motor shut down.