Torque limit

[bob]

Hi,

 

We have actually installed one 900 k W motor driven by a VSD , and this drive runs two independent rollers but with only one main gearbox. We are having regularly gear damage on one of the roller, because one roller could at a particular time take the full 900 k W torque available at the output shaft of the gearbox. The drive is set for V/F application.

The obvious solution would be to have independent drives for each roller, but this solution is too expensive for us for the time being.

I am thinking of one torque limiting device !!!!!!!!! attached to the shaft of each roller so that we could trip or stop the VSD if the torque exceeds a particular limt.I am wondering if such a device exists.

I am welcoming suggestions from orum members.

 

Bob

[Subhashish]

Shear pin built for a defined torque is an reliable device which breaks on overtorque and detaches the driven assemblly from the driving mechanism . However Iam not sure of the exact mechanical arrangement of your machine and hence its difficult to state if a shear pin arrangement can be made .

 

Another means is to provide a torque sensing assembly to the rotor and electronic signal from the torque sensor can be used to trip the VFD on over torque .

 

Most of the electronic drive also reads the torque delivered .

You can set the torque limit and drive shall trip on overtorque condition.

 

Regards

[bob]

Yes, the drive is reading the total torque delivered that is the torque of both rollers together. It is quite difficult and I think more expensive to attach shear pin arrangement to the shaft.

 

Bob

[Subhashish]

that's true . torque at individual roller needs to be sensed .

May be torque sensor should be a good solution .

Can check with FF420 torque transducer of datum electronics ( both contact and non contact types are available ) or may be many more similar kind of sensors available with other manufacturers .

Out put is in form of 4-20ma or 0-10 volts , something that you can comfortably process to serve your purpose .

 

Regards

[jraef]

I agree. That is really your best option since the VFD will not be able to determine torque sharing below the gearbox. The only other course in my view is to determine the cause of such a severe imbalance in load sharing and try to detect the cause or another effect of that condition. That may not be as reliable however. If you can retrofit a torque sensor, that would be better.

 

Here is one I have used in the past on a test bench application. I'm not sure if it is meant for production machinery, but you could ask them.

http://www.hbm.com/products/SEURLF/ASP/SFS...ctDataSheet.htm

[marke]

Hi Bob

 

One solution is to fit a fluid coupling to each roller. - a simple mechanical means of limiting the torque.

 

Best regards,

[Subhashish]

Hi

 

Fluid coupling to my understanding are used for equipment that needs large starting torque like Centrifuge and polymer powder homogenisers rather than for limiting overtorque .

 

Fluid coupling do operate as a protective device in case of overtorque by operating the fusible plug and splashing out oil thus disengaging the coupling between motor and the driven equipment . Its generally a very undesired situation and I don't think that this shall be comfortable way of protecting .

 

Also retrofitting a fluid coupling may not be very easy.

 

Regards

[marke]

Hi Subhashish

 

Yes, the fluid coupling is often used as you described because it limits the troque transfer. It is a torque limiting device that does not trip, if the torque gets too high, it slows down.

The fusible plug operates, not on torque, but on temperature. If the torue is excessive for a long time, the slip losses will cause too much emperature rise and this will blow th plug.

 

Best regards,

[jraef]

I think Marke makes a good popint I hadn't considered. Being an electrical guy, I tend to look for electrical solutions.

 

There are mechancial means of protecting the components. While I tend to think the fusible plug fluid coupling is too messy, it s probably easier to clean that up than to replace the roller. There are also non-fluidic devices called Torque Tamers that would do essentially the same thing, but work on a principle similar to an automotive clutch. Two plates and a fiber disc designed to slip at a specific speed (edit: should have said torque). If you can istall something like that, you can then add a simple speed detection system to the rollers so that if one starts to slip, you can shut down the drive.

[Subhashish]

yes ,friction couplings gets pneumatically dis engaed when it gets signals from slip detectors . Such couplings are engaed at particular pneumatic pressure and the slip sensor is placed on either side of the coupling .

 

Any way which ever method is used retrofitting shall involve cost and difficulty but I agree that this is may be more cleaner than fluid coupling .

 

regards

[bob]

Hi all,

 

Tell me, basically how is the torque sensor mounted? I believe my system should be retrofitted to fit the sensor. I am talking shaft diameter of about one metre.

I have requested quotations from Datum electronics and some others but without success.

 

Bob

[Subhashish]

hi

bob

 

Torque sensor can be fitted into any size of shaft , can you give a bit more description about the arrangement of rollers? its basically difficult to visualise the arrangement of rollers from the brief description of system you have provided .

 

is any one circular face of the roller free for retrofitting the torque sensing assembly ? Is it possible to make bore at the circular face cetre of roller to attach the the torque sensor assembly via load beam and gear assembly ? Such sensors can be set to required torque value and can generate trip command if the value is crossed .Also 4-20ma monitoring signal is abailable

 

Regards

[Guest]

Hi Subhashish,

 

The 900 k W motor drives a reduction gear box to which is coupled a main gear wheel. Geared to the latter are two other gear wheels which drive two independent rollers. Basically, it should be fitted to the shaft driving the independent roller.Could it be mounted at the shaft end ?

I believe the sensor should be mounted between two seperate shafts .

 

Bob