Brake Shaft Bending: Causes & Gyroscopic Effects

[bhaazee]

I read an article about Brake Shafts bending behavior due to braking.

Assume that there is a Shaft and to its end a brake disc is fixed. The shaft is set to rotate at certain rpm. Now when brakes are applied it seems the shaft tend to bend.

Could anyone explain me the reason for this behavior. Has it anything to do with gyroscpic effect? I personally have no idea?

 

[berkeman]

I read an article about Brake Shafts bending behavior due to braking.

Assume that there is a Shaft and to its end a brake disc is fixed. The shaft is set to rotate at certain rpm. Now when brakes are applied it seems the shaft tend to bend.

Could anyone explain me the reason for this behavior. Has it anything to do with gyroscpic effect? I personally have no idea?

Welcome to the PF.

Disk brakes are generally not attached by shafts to anything. Do you have a link to the brake geometry that you are asking about?

 

[AlephZero]

If you post a link to the article, we can try to explain it, or tell you why we think it is wrong.

Otherwise, the question is too vague to be answerable. Without a drawing, I don't have any idea what type of braking system you are talking about.

 

[bhaazee]

Well sorry for my late reply and Thnx for the replies.

I don't have a proper cad model or image of the project. I can't provide you the link as it is some research project at my univ.

I will try to explain you in a clear manner.

I have a long shaft rotating (driven by a motor) about say x- axis. And the braking disc of the hydraulic disc brake is attached to the end of the shaft. This means the shaft with the braking disc tend to behave like a cantilever beam. the shaft is stiff enough to resist the bending (the bending due to itself weight and the weight of the hydraulic braking disc). now the shaft is rotating at 'x' rpm. when I intend to stop the shaft from rotating I apply the brakes and the calipers in the brakes applies a tangential force on the braking disc (in the direction of rotation). Now it seems a downward force is created on the rotating shaft.

Thus means the total force which tends to bend the shaft in the direction of Earth's gravity is
Weight of the shaft+ weight of the braking disc+ the downward force creating during braking.

Now I just want to understand what creates this downward force during braking.

Now I hope you can understand it still better.

 

[AlephZero]

Without knowing any properites of the system like dimensions, mass and inertia, speed, and the deceleration rate for your braking, this is really just guessing possible causes - but they are both "real" effects that can occur with the right conditions.

1. The shear force of the shaft's own weight, plus the torque when it decelerates, combine to create an unsymmetrical stress distribution in the shaft with a higher stress on one side of the shaft than the other. This could be deforming the shaft so the braking force also becomes unsymmetrical around the circumference. In extreme conditions, you can finish up with plastic deformation of the shaft into a helix shape. If the plastic yielding is local rather than global, it may look like a "plastic hinge" between two straight sections of shaft.

2. Whirling. In most situations, the most interesting (i.e. troublesome) whirling modes are the synchronous forwards whirl modes, which are excited at specific RPMs because the shaft is not perfectly balanced. However, at any shaft speed there are potential non-synchronous whirling modes that can travel in either direction relative to the shaft rotation. (Forwards whirl means whirling in the same direction as the shaft rotation, backwards whirl means whirling in the opposite direction.) Friction forces can excite backwards whirl modes. These can be potentially unstable, if the deformed shape makes the friction force unevenly distributed round the disk. The unsymmetrical force distribution also gives a resultant shear force. A typical situation would be that the brakes "grab" or "bind", the rotor stops turning very quickly, and most of the energy goes into a cantilever vibration mode of the rotor.

To add an obvious (well, obvious with hindsight and past experience!) comment the dynamics of the system: the vibration and whirling modes may look very different when the brakes are on or off, since the brakes will form another "bearing" on the rotor system.