Failure Shaft

There is no way anyone can give you the root cause from these photos. It may well be that bending has caused the failure but what caused the bending? I would check you understand the meaning of root cause first.

The cause of the failure was the inability of the shaft to withstand the stresses that were imposed on it.

Of interest are the circumstances surrounding this overload, which cannot be seen from here.

The clean concave (on the gearbox end) break leads me to believe shock loading.

Please give some other details like service, loading frequency , type of drive, RPM, service temperature etc.

Looking at the clean convex end on the one shaft almost appears as if a guard had slipped loose and was scoring the shaft and reducing it's strength. Once a heavy jam occurred, what was left could not withstand the torque. That's conditional whether a guard was present or not.

A possibility anyway.

Are you sure that you do not have an old shaft repair failure?The pictures appear to indicate stark delineation of three very different metals which could be resultant from a past repair wherein the shaft was "sleeved". (Out side shaft failed, both peices were bored out, and a new section of smaller shaft inserted then welded/secured.) Bandaid repair at it's best!

Or:

Maybe the pulley was originally mechanically designed with the shaft outside diameter set as the larger size. The original pulley failed, a replacement pulley with a smaller inside diameter for the drive shaft was installed, and the original larger O.D. shaft was cut down to fit the new pulley and it simply did/does not have enough mechanical strength to meet the mechanical stress placed on it.

Or:

The bearing clearly indicates lack of preventative maintenance and severe mechanical overload abuse. Maybe the shaft bearing failed, seized, and then the centrifugal inertia from the load being driven was greater than the mechanical design limit of the system causing the driven to become the driver.

How's that for thought provoking guesses?

Seriously;

In most cases equipment is mechanically altered over it's lifetime and eventually the changes cause catastrophic failure of a component or occasionally, the entire system.

Find and review the original mechanical design specifications for the application. (shaft size, shaft metal composition, rockwell hardness - too soft or too hard, pulley diameter, etc.)

Compare each and every existing mechanical component to the original spec.

Look for system dynamics and operation outside the design specifications.

Check and verify driver (motor) horse power rating does not exceed the original mechanical design specification.

If equipped with an arresting or braking system: Verify the arresting system is functioning correctly. (Application time delay period correct, electrical regeneration dissipation circuit working correctly, brake pads/shoes and rotor/drum compostition correct, etc.)

If all of the above components are within tolerance and design specs, and the unit has been in operation for an extended time period, you need to look at mechanical abuse, operational abuse, and lastly chemical impingement of the equipment.

A word of warning: Many times the equipment RCA report yields information not well received by management so be careful how you present your arguement and make sure you have documentation to justify your findings.

Good luck, looks like you have your work cut out for you.

Jim

"In order to eat an elephant you must eat one bite at a time just as you would if you were eating an apple." ?

If the failure was excessive torque, a torn surface with a radial "wave" often results. (put a bolt in a vice and break it by overtorqueing it with a large spanenr to see the surface I mean), so I don't think that this was the cause.

The failure here is a brittle failure as seen by the grainy finish. The material is brittle as seen with high strength steels and say cast iron (unlikely for this application).

Often with fatigue failures a considerable portion of the shaft is cracked before the final failure. The large area (more than half) that is the final brittle failure here shows that there was a heavy load on the shaft when it failed (probably not much torque).

What caused the failure is uncertain, but certainly it was highly loaded (low safety factor). A factor of safety of 2 (failed over half the area) would be pretty low for this sort of application.

I hope that helps.

There is also a large area around the brittle fracture that has seen considerable working, and it appears some corrosion so it may have been damaged for some time.

I would be suspicious of a stress riser (insufficient radius) between the shaft and the drum face. This may have caused a crack that worked for some time, until the shaft diameter supporting the roll was insufficient and proceeded to the brittle fracture.

So much for my blowing smoke, it just looks like a shaft that failed on me and that was the explanation given.

The broken shaft looks like a push and pull rod with quadrant enclosed in a bigger shaft,applicable for ship propulsion with controllable pitch propeller.The probable cause may be small clearance within this two shaft probably due to ingress of sea water from outside if propeller blades seals become defective.In that case the push and pull rod will not move freely.May be during repair, one way or other some one applied excess force say with jack than required then he may break the rod.

Please give following information before I can give my comments:-

1. Application where shaft is used, machine for which it is used.

2.Duty of machine is it subjected to shock loads.

3. Picture/sketch of original shaft.

4.Heat treatment process.

This is nothing but poor metal Composition,you need to check the mettallurgy.