Reasons for Shaft Shearing

Posted in: , on 24. Feb. 2012 - 03:58

Dear Experts,

In the last few months, we have faced few shafts shearing at different equipments. Two were at two different tripper conveyor's long travel wheels' shaft. One was a drive pulley shaft of a conveyor. The main problem is such failures are happening without any prior indications, and very time consuming. Wheels, bearings, adapter sleeves etc, may be considered as spares, because they are subjected to wear & tear. But shafts were not considered as spares.

What all could be the reasons for shaft shearings? How can they be avoided in future? Can such failures predictable in advance? Please reply.

Thanks & Regards to all,

Re: Reasons For Shaft Shearing

Posted on 24. Feb. 2012 - 10:08

Dear sganesh,

Have you examined the cause of the shaft shearing?

Did the shafts fail because of bending. torsion or plain shearing?

Have you examined the failed surfaces, in order to find out whether the failures were induced by fatigue or acute overload?

Have you recalculated the shafts (both on strength and fatigue), based on the actual loadings, investigating the design of the shafts?

About predicting failures, it is obvious that when shafts fail, you can expect future failures.

In case of fatigue, cracks are developing in time, but probably in places, which are not always visible.

A mechanical engineer with knowledge and experience is recommended.

Success

Teus

Teus

Reasons For Shaft Shearing

Posted on 26. Feb. 2012 - 04:28

In my experience there are only two reasons for failure; under-design or overload. Actually, both are the same. The design is not adequate for the load. Sometimes it is faulty analysis of known loading and duty. Sometimes it is good analysis of what is known but a failure to know the actual loading and duty. Inspection of the failure to determine its nature will reveal the cause and guide you in a more intelligent, more targeted re-analysis.

As a civil-structural engineer turned mechanical I am often critical of how many mechanical engineers will turn to ever higher steel strengths to make their designs work within the increased allowable stress. High strength come at a sacrifice of ductility and margin against rupture. Fatigue strength is often dependent on profiling (machining), surface finish and surface treatment.

Just some thoughts. Analysis of the failure will reveal the problem and guide you to the correct solution.

Joe Dos Santos

Dos Santos International 531 Roselane St NW Suite 810 Marietta, GA 30060 USA Tel: 1 770 423 9895 Fax 1 866 473 2252 Email: jds@ dossantosintl.com Web Site: [url]www.dossantosintl.com[/url]

Re: Reasons For Shaft Shearing

Posted on 26. Feb. 2012 - 05:42

Dear Mr.Joseph A. Dos Santos & Mr.Teus Tuinenburg,

I understand from your replies that the analysis part is possible after the break down.i.e, the shaft was sheared.

Also when a shaft is subjected to overloaded, why the motor should not trip? So, shearing process would have been started since long time, so that motor protection could not come in picture during the break down. I am looking for any tool or guidance to make sure that the shaft is designed & manufactured so that failure could be predicted or avoided.

Thank you very much & a lots of regards,

Re: Reasons For Shaft Shearing

Posted on 26. Feb. 2012 - 06:16

Dear sganesh,


I understand from your replies that the analysis part is possible after the break down.i.e, the shaft was sheared.

Indeed, the failed surface indicates the nature of the stresses and loads that led to the failure.


Also when a shaft is subjected to overloaded, why the motor should not trip?

The motor is loaded with the shaft torque only.

Excessive bending stresses are not loading the motor, but can cause (fatigue) failures.

If you can show us a macro picture of the failed surface, a better answer (or even a conclusion) can be given.

Again, read the previous posts and check the design against the applied material and the field operations.

Take care

Teus

Teus

Roland Heilmann
(not verified)

Shaft Failures

Posted on 27. Feb. 2012 - 12:46
Quote Originally Posted by sganeshView Post
Dear Mr.Joseph A. Dos Santos & Mr.Teus Tuinenburg,

I understand from your replies that the analysis part is possible after the break down.i.e, the shaft was sheared.

Also when a shaft is subjected to overloaded, why the motor should not trip? So, shearing process would have been started since long time, so that motor protection could not come in picture during the break down. I am looking for any tool or guidance to make sure that the shaft is designed & manufactured so that failure could be predicted or avoided.

Thank you very much & a lots of regards,

Dear Mr. Sganesh,

please be aware that shafts as any other part in your systems have been designed to a certain "status quo", meaning, that if your system did undergo any changes, life expectancy of it's parts will change too. Such changes might be accumulated misalignment of rails (sic travel drive shafts) or poor adaptability of a selfaligning roller bearings (if any?!) of a drive pulley bearing or general misalignment of those pulley bearings. IMhhO it's not generally one address that can be blamed, and it's most counterproductive to do so already before analysis. Best way: Invite the (reputable) supplier of those items, even if warranty has expired.

By the way, anything that exists has its design life span, shafts surely not excuded. There's perhaps a rather high price on things that would hold forever, and lets see the warranty period for those.

Then, purely technical, the inner stress of a shaft is defined by lots of parameters, where torque input is just one part of the whole picture. Analysis of the broken shaft will show, whether the failure was due to a single overload event or perhaps fatigue at a stress raiser zone. So I'd join the collegues above in asking for some photos and more detailed information.

Regards

R.

Re: Reasons For Shaft Shearing

Posted on 28. Feb. 2012 - 09:01

The shaft is experiencing pure torque. There are no stress risers in the shaft. Multiply your result by the stress riser value to get your actual shaft shear stress.

Re: Reasons For Shaft Shearing

Posted on 28. Feb. 2012 - 02:34

Dear ftracker,


The shaft is experiencing pure torque.

How did you reach this conclusion?

The thread start clearly states:


two different tripper conveyor's long travel wheels' shaft

and


a drive pulley shaft of a conveyor

“Travel wheels' shafts” have certainly bending stresses.

“A drive pulley” should have a bending moment too, otherwise it should be named a coupling half.


There are no stress risers in the shaft.

Multiply your result by the stress riser value to get your actual shaft shear stress.

If there are ”no stress risers in the shaft”, should the ”stress riser value” not be 1?

Best regards

Teus

Teus

Re: Reasons For Shaft Shearing

Posted on 14. Apr. 2012 - 07:33

Hello Shri S. Ganesh,

Shaft shearing can occur due to:

A) Shaft actual occurring shear stress are excessive with respect to yield stress and endurance stress limits.

B) Radial compression hub and shaft is not proper (too high or too less (nil) that is loose). The loose fitment will create knocking or impact loading in every revolution.

C) Required shaft size should be calculated considering stresses as well as deflection. Cross section change should not be abrupt by large margin (should have proper taper).

D) Workmanship is not proper about concentricity of hub bores and fitment tolerances.

E) Difference in required quality as per design and actually made pulleys.

The above can be expressed in varied manner. The readers can add their views.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916

Email: parimul@pn2.vsnl.net.in

Reasons For Shaft Shearing

Posted on 18. Aug. 2012 - 03:36
Quote Originally Posted by joseph a. Dos santosView Post
in my experience there are only two reasons for failure; under-design or overload. Actually, both are the same. The design is not adequate for the load. Sometimes it is faulty analysis of known loading and duty. Sometimes it is good analysis of what is known but a failure to know the actual loading and duty. Inspection of the failure to determine its nature will reveal the cause and guide you in a more intelligent, more targeted re-analysis.

As a civil-structural engineer turned mechanical i am often critical of how many mechanical engineers will turn to ever higher steel strengths to make their designs work within the increased allowable stress. High strength come at a sacrifice of ductility and margin against rupture. Fatigue strength is often dependent on profiling (machining), surface finish and surface treatment.

Just some thoughts. Analysis of the failure will reveal the problem and guide you to the correct solution.

Joe dos santos

the notch effect due to step formation on the shaft at both ends may also contribute to shearing of shaft particularly at the step. There should be a conical step instead of 90 degree step at the ends.

Regards

ravindranath b ( raviinven123 )

Reasons For Shaft Shearing

Posted on 18. Aug. 2012 - 03:57

Mr. raviinven123,

Indeed, design of the turn-down will determine the stress riser. These factors can be found in a Machinery handbook. The case is similar at a keyway if a keyway is used.

Joe Dos Santos

Dos Santos International 531 Roselane St NW Suite 810 Marietta, GA 30060 USA Tel: 1 770 423 9895 Fax 1 866 473 2252 Email: jds@ dossantosintl.com Web Site: [url]www.dossantosintl.com[/url]

Shaft Calculation

Posted on 30. Aug. 2012 - 03:43

Would you please specify below items for the shaft:

1- power transmit

2- velocity rpm

3-shaft diameter in the failure point

stevenwang
(not verified)

Re: Reasons For Shaft Shearing

Posted on 7. Dec. 2012 - 04:20

The shear stress in a solid circular shaft in a given position can be expressed as

= T r / Ip

where

= shear stress (MPa, psi)

T = twisting moment (Nmm, in lb)

r = distance from center to stressed surface in the given position (mm, in)

Ip = "polar moment of inertia" of cross section (mm4, in4)

The "polar moment of inertia" is a measure of an object's ability to resist torsion.

Reasons For Shaft Shearing

Posted on 7. Dec. 2012 - 03:30

Mr. StevenWang,

Indeed that is the torsional shear stress formula for a shaft subject only to torsion. The actual loads are typically torsion, direct shear and bending moment. These have to be superimposed for max shear and max principle stress in a Mohr's circle analysis. Furthermore this assumes uniform load application and there are no steps, keys or any discontinuities in the shaft. Analysis of any actual shaft requires all of the information and then one can use the appropriate tabulated stress riser or one can perform a better analysis using more sophisticated methods such as FEA, photo elastic, etc. That is just the analysis part. What is the shafting material? What is the yield stress and ultimate rupture stress? What is the ductility? What are the surface finish and treatment and the corresponding stress allowables.

This is not intendend to scare anyone away from the analysis. It is merely to urge you to be thorough and carerful and to look up and address all relevant issues.

Joe Dos Santos

Dos Santos International 531 Roselane St NW Suite 810 Marietta, GA 30060 USA Tel: 1 770 423 9895 Fax 1 866 473 2252 Email: jds@ dossantosintl.com Web Site: [url]www.dossantosintl.com[/url]

Re: Reasons For Shaft Shearing

Posted on 10. Dec. 2012 - 04:32

Joe

Does conveyor01 ring a bell? I won't say anymore. I am sure you know what I mean.

Gary

Gary Blenkhorn
President - Bulk Handlng Technology Inc.
Email: garyblenkhorn@gmail.com
Linkedin Profile: http://www.linkedin.com/in/gary-blenkhorn-6286954b

Offering Conveyor Design Services, Conveyor Transfer Design Services and SolidWorks Design Services for equipment layouts.

Randy S
(not verified)

Pulley Design

Posted on 15. Dec. 2012 - 04:22

Whats unknown is where did the shaft break. Its a drive pulley so there are a few questions needing answered prior to to having a direction to investigate

- Did it break outboard of the bearing? If so is it a rigid coupling shrink style mount? hollow shaft mount? Was it related to the overhung load of the gear reducer and motor, or was it inboard of the bearing under the belt load plus torque?

- Did the shaft break at a turndown? Was it a sharp turndown, what was the surface finish?

- Did the shaft break at the leading edge of a tapered adaptor sleeve or edge of a locking device? if so it may be from fretting corrosion and indicate poor design in regards to accommodating bending moments.

- Does the break look like a classic fatigue failure originating from a crack at one edge and propagating inward until finally fracturing against the opposite side. Or does the final fracture look like a circular area more towards the middle?

As you can see a simple "why does a shaft shear" can be as complex as why does my car not start.

Randy Smyth

Reasons For Shaft Shearing

Posted on 15. Dec. 2012 - 06:20

Randy,

All very good points. Please note that the original post expresses concern with three cases shaft shearing, two at tripper travel wheels and one at a drive pulley.

Joe Dos Santos

Dos Santos International 531 Roselane St NW Suite 810 Marietta, GA 30060 USA Tel: 1 770 423 9895 Fax 1 866 473 2252 Email: jds@ dossantosintl.com Web Site: [url]www.dossantosintl.com[/url]