# Force required to stop a given torque

Hi Guys,
I am am trying to find out the force required to hold a piston in its place without rotating during an assembly process. The torque applied on the piston is nearly 50 Nm and the wrench has a length of 250mm. Would T-rfsin(theta) solve my issue? or is there something else I would have to take into account. Thanks.

I am using a pneumatic piston to exert the force

russ_watters
Mentor
You can't apply torque to a piston, only linear force. And what is theta?

Can you provide a sketch of the mechanism? This migh be a standard linkage problem and your approach might be correct but I can't tell.

Last edited:
You can't apply torque to a piston, only linear force. And what is theta?

Can you provide a sketch of the mechanism? This migh be a standard linkage problem and your approach might be correct but I can't tell.
Hi russ,
Thank you for the reply. I am just working on the process of assembling an oil pump that has a piston moving up and down in it. However the torque that i have mentioned here is not regarding the funtionality of the pump but on how the pump is actually manufactured/assembled.

In the process we pneumaticaly clamp the piston on its O.D into an assembly fixture using a rubber jaw. We do this so that we could tighten some other components onto the piston top at a certain torque using a torque wrench while the pneumatic clamp holds the piston stationary.
I need to know what amnt of force is required to hold the O.D of the piston in the fixture without rotating when using the torque wrench or in other words the pneumatic clamping force required to overcome the rotation of the piston at a certain torque during tightening of components above the piston.

Is the piston to clamp surface lubricated or dry? Sound like you will likely need the coefficient of friction between the piston and the clamp, and thatis something best determined experimentally.

CWatters
Homework Helper
Gold Member
The torque required to stop the piston rotating is the same as the applied torque (call it T). If fitting is being bolted to the centre of the piston and the piston was of radius r then the tangential force required on the circumference of the piston is T/r. To find the clamping force you need the coefficient of friction as Dr.D said.

CWatters
Homework Helper
Gold Member
Is the gudgeon pin hollow? Could you put something through it to stop the piston rotating rather than clamp it from the outside?

If the piston is a casting perhaps cast in something on the inside and make a tool that mates with it so that the force is on the inside of the piston rather than the critical outside surface.

Dr.D and berkeman
Hi russ,
Thank you for the reply. I am just working on the process of assembling an oil pump that has a piston moving up and down in it. However the torque that i have mentioned here is not regarding the funtionality of the pump but on how the pump is actually manufactured/assembled.

In the process we pneumaticaly clamp the piston on its O.D into an assembly fixture using a rubber jaw. We do this so that we could tighten some other components onto the piston top at a certain torque using a torque wrench while the pneumatic clamp holds the piston stationary.
I need to know what amnt of force is required to hold the O.D of the piston in the fixture without rotating when using the torque wrench or in other words the pneumatic clamping force required to overcome the rotation of the piston at a certain torque during tightening of components above the piston.
Is the piston to clamp surface lubricated or dry? Sound like you will likely need the coefficient of friction between the piston and the clamp, and thatis something best determined experimentally.
[quote fixed by mod]

Hi Dr. D,
The piston is anodized on the outer surface and is not lubricated... however having a smooth surface finish makes it critical to hold the piston just enough without damaging the surface.

#### Attachments

• 15294959674421140446255.jpg
21.3 KB · Views: 265
Last edited by a moderator:
Is the gudgeon pin hollow? Could you put something through it to stop the piston rotating rather than clamp it from the outside?

If the piston is a casting perhaps cast in something on the inside and make a tool that mates with it so that the force is on the inside of the piston rather than the critical outside surface.

Hi CWatters,
We used to clamp the piston using a through pin since there is a hole but we are in the process of making a universal fixture that fits all our pistons of different pump variants and not all of them have a through hole and moreover the dimensions of hole vary making it hard to clamp withput using an adapter pin for our fixtures

We used to clamp the piston using a through pin since there is a hole but we are in the process of making a universal fixture that fits all our pistons of different pump variants and not all of them have a through hole and moreover the dimensions of hole vary making it hard to clamp withput using an adapter pin for our fixtures

If the external finish is critical, as it appears to be, then an adapter pin for each size piston seems like a far better solution. In any event, you are simply trading off one problem for another.

bijoy Thomas