Broken Tail Pulley Shaft 2

[Vibroman]

Couple of days ago we had a failure on one of our conveyor's tail pulley shaft.
I added a picture of surface. The speed of the pulley is approximately 120 rpm.
I did a little bit search and it looks like rotating bending fatique? Can anyone who is experienced on failure analysis help me on the type of failure.
Thanks,

 

[Vibroman]

Just an additional information, this pulley also has a take up system, which means we are tightening the belt with moving the frame which is underneath the pulley bearing housing.
The red frame in the attached picture. It is done simultaneously from both housings. I'm not sure if too much tihtening played a role here,

 

[MikeHalloran]

Let's start with the assertion that everything has a fatigue life.
How many decades did the shaft survive before fracturing?

As for overtightening, it happens, but usually not with experienced millwrights, unless the design was wrong from the start.
As an engineer, you can estimate the power required to drive the conveyor, turn that into belt tension, and do a free body diagram on the shaft.
The most obvious general question is the distance by which the drive pulley extends past the closest shaft bearing; ideally it's zero, but a couple of shaft diameters is usually OK. I can't guess anything about the shaft arrangement or the belt drive from photos provided so far.


Mike Halloran
Pembroke Pines, FL, USA

 

[lukin1977]

the shaft broke just after the bearing? were both sides tighten equally?

 

[gearcutter]

Judging by the rough & faceted fracture surface, a very short time elapsed from crack initiation to ultimate failure. That fracture surface had very, very few load cycles to failure. I would view this as a brittle fracture and would start looking for the mechanisms behind such a sudden overload.

 

[Vibroman]

Mike,
This was a newly installed conveyor, it just run 1,5 months and then we have faced with this failure. To be honest i don't think it is a design issue, thats why i didn't go through with the calculations. But i will for sure now.
Lukin
I added another photo, it is broken just from the pulley to shaft bushing. I'm not sure if they were tighten equally, this happened in a weekend and guys replaced the pulley immediately. There is a big chance that operation group tightened them unequally...
Gearcutter
thanks for the information,we started to think that too much tightening is the issue here. I'm sure it is not fair to ask this question but When you say very very few loads are you talking about hours?

Thanks to everyone

 

[gearcutter]

When you say very very few loads are you talking about hours?

No, I'm talking about load cycles. In this case..........rpm under load.

 

[gearcutter]

I would post your images over in the Metal & Metallurgy forum -

http://www.eng-tips.com/threadminder.cfm?pid=330

Keep the question you post specific to the fracture zone and its appearance.
Be careful that you do not copy this post and then re-post in other forums as this is not allowed, it's called cross-posting.

 

[Vibroman]

ı moved it,

Link

Thanks,

 

[Tmoose]

Unless the pulley/sheave loosened on the shaft I would not focus too much on the taper lock bushing bolt/screw tightening.

Was this a new machine, or reconditioned, or just a "good used" machine.

 

[dvd]

I would recommend to call PPI. They made the pulley and probably can tell you quite a bit about whether they are going to replace it.

 

[Strong]

Did the bearing seize causing torque overload and twisting shaft?
Walt

 

[tbuelna]

From the last photo provided it appears the shaft fractured inboard from the end of the split taper bushing. May have been some misalignment between the taper bushing and shaft surface that caused sufficient damage to initiate a fracture given the 1/rev reverse cyclic bending load applied to the shaft.

 

[RolMec]

The seat of the taper lock at the hub / shaft connection is a known area of concern for crack development / failure with belt conv. pulley shafts.
As for fatigue, assuming just 10/24 and 6/7, we look at +2,5*10⁶ cycles and are well placed for relevant failure. Pls. see attached.
As for overtightening, I would not think that anyone should have been retightening the taper lock device, pulleys being normally delivered as an ready-to-use integral unit of pulley shell + shaft + locking devices. This is, at 1,5 month from delivery, entirely within the scope of mfgrs warranty.
As for checking, this calls for a full blown investigation into
- loadbearing capacity of the shaft zone affected, considering all stress raisers
- metallurgical exam & crack analysis
- load amplitudes and cycling characteristics
- operations (extraordinary events, op. history until failure)
- design environment and influences from pulley bearing & support design
and check this back against the design calculations.

No one should accept to work with a system where such failure might occur again & at random. And from a legal p.o.v. you are now "in" to know there's a weakness which may have severe effect at least for operations material , if not for the operators. What if the belt does really start to burn & starts a fire, which is a real (in my prof. experience) risk with such failure mode?
In belt conveyors, the rule is: "Protect the belt", it's the major item in capex.
Regards
R.

RSVP

 

[RolMec]

From a feedback I just got, pls. put a very meticulous effort on the analysis of the taper lock device & its surroundings (local bending, gaping [I don't know the correct expression] due to bending action, susceptibility to coming loose due to bending / deformation). A hallmark to look for is freeting corrosion.
Best of luck for you to find the root cause!
It would be very kind to offer your findings & conclusions to the forum!
Thank you in advance,
Kind regards

R.

RSVP