Maybe, but where's the formula? ■

Heavy ..............

Plugging numbers into a long complex formula leads to mistakes and a lack of understanding of how something is done.

You really need to work out basic shaft design from first principles.

Draw out the shaft and add the loads at the correct positions.

Work out the torque being transmitted and the bending moments at the critical positions.

Select the material you will use for the shaft and establish is maximum tensile and shear stress, then choose a suitable factor of safety.

Choose what you consider is the most suitable method to combine the torque and bending moment with the stress to calculate the shaft diameters.

I generally combine the torque and bending moment into a combined bending moment

Mcombined = M + Sqrt(M^2 + T^2)

then work from

Bending stress = Mcombined / Z

All of this is basic engineering stuff, no rocket science. If you do it my way you'll understand what you are doing.

Hope this helps.

PS as regards the Excel formula I would suggest 32 comes from the calculation for Z ■

Are you actually moving and ore from point A to point B and if so how much. ■

?????? ■

Designer's method is a lot easier thna faffing about with a formula showing more brackets than a short conveyor.

Don't forget to check the keyway influence as a final step. The net shaft diameter must transmit the torque, for the easiest. ■

Forgot that bit. Rule of thumb with ISO keyways, add 10% to shaft diameter without keyway. ■

I think you need to take a look at Shigley or Roark. It appears you are not an engineer and should seek some guidance.

THe value 32 comes from the bending stress formula. Where the bending stress (sigma) = (M) (c)/(I). (M) is the bending moment. Where (I) is the moment of inertia of a cylinder or shaft . (I = Pi x (diameter ^4)/64). Where (c) is the distance from the shafts neutral axis (c = d/2). When (I) is divided by (c) then 64 becomes 32.

The value 1.5 is a bending stress fatigue allowance factor for momentary overloads such as impulse starting or stopping (quick braking). There are other factors.

Beware if you have turndowns between shaft hub and bearing.

Beware of the different locking mechanisms connecting shaft to pulley.

Beware of overhung loads and so on.

If your life or friends of the family lives depend on your analysis and its application, get help before the bells toll. ■

All you have to do is calculate from first principles using standard formulae like prescribed by Larry, and ensure that you limit shaft deflection to about 5 minutes as calculated at the hubs, and limit the stress to about 55 MPa.

God luck

LSL Tekpro ■

First principles is good.

I don't know about Shigley, but with Roark you can still end up putting numbers into a formula you don't understand (been there, done that ..).

Limiting angular deflection is fine, but doesn't a working bending stress depend on the quality of the steel you are using? ■

For non drive pulleys use EN3A and for drive pulleys use EN9

I use 5 minutes and 55Mpa for both drive and non-drive, and Bob's your Uncle.. life is easy...no pulley failures in 36 years.

Cheers

LSL Tekpro ■

Yes, it does depend on the quality of steel. The value 16500 is in psi and sets the minimum allowable bending stress in your formula. This is a mild steel value.

Again, you should not practice sticking numbers into things you do not understand where life and limb are at risk.

By example, a well respected mine maintenance manager, of a major world class mine, made a weld repair on a large drive pulley shaft when a bearing seized and scored the shaft. The repair looked good. The shaft failed within weeks of putting it back into service. The weld annealed the steel and brought the allowable stress limit to below the applied load. The failure cost the mine tens of millions of dollars in lost production. Luckily, nobody was hurt by the flying debris. The debris failed the conveyor and the parallel conveyor adjacent to it. He certainly should have known the consequences of the weld and should have made a analytic check of his action. ■

Your age is showing .............

I was once told by an experienced conveyor guy don't use EN24T for shafts as you'll find the size is determined by deflection and you'll waste an expensive steel. Use EN8 and the size is determined by the stress, and it's a cheaper steel to buy. ■

Exactly so!

LSL Tekpro ■

The 'EN' designation was a range of steels to British Standard 970:1955.

I'm not sure when BS 970 changed from these designations, but it was a long time ago (EN24T became 817M40T).

But engineers can be a conservative and there was something nice about 'EN8' and 'EN24' so the designations are still widely used if out of date! ■

Dear Mr. LET123,

The shaft is being designed by the engineers since about two centuries. Therefore, shaft design formulae are a well-known text book affairs. The important issue is to find out the forces acting on the shaft, in context of particular machine or some item, where the shaft is to function. Apart from stresses, one has to also see the allowable deflection in particular application. The safety factor value will be commensurate to the sophistication of design and the quality of material and manufacturing.

If you have a very simple / small conveyor and very simple pulley, one can design the shaft for the pulley as per the procedures given in conveyor literature.

The shaft forms a part of the pulley where shaft design is influenced by the complete pulley design, because this is composite structure. Pulley design procedure and thereby the shaft design, is a very complicated calculation and generally it should be left to the regular designers of the pulley.

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.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Designer..

For what it is worth, here in South Africa we actually still get by calling steel EN3A, EN8, EN9, etc. and EN terminology is commonly used in our clients specifications too.

Using the above with the deflection and stress limitations discussed previously, sleep comes easily.

Regards

LSL Tekpro ■

Alas I've never written a book, or got strings of letters after my name and it's too late now ....... :-((( ■

Don't alas lad. Each letter costs about 20 quid every year: so think of the money you're saving while still being a clever bod. ■

Would you be from Yorkshire ;-) ■

Shaft design as can be seen by some of the above comments is a complex area and one that should not be entered into lightly. Safety should be high on peoples priorities as pulley shafts breaking are a dangerous thing to happen.

As a designer of said pulley shafts I prefer to work from first principles on shaft design, however, various eminent bodies have done a lot of work on this subject and come up with a set of standards that can aid the average pulley shaft designer.

I would like to refer designers of pulley shafts to the following standards/codes which I am sure most people will be familiar with or have some working knowledge of. These standards/codes give a good guidance to a pulley shaft diameter for both driven and non driven shafts.

ANSI/ASME B106.1M

Australian Standard AS1403

DIN 743

Good luck with your design. ■

Paul... et al..

What people must realise is that it is no use whatsoever in calculating many pulley shafts to 3 everso-clever rocket science decimal places, and basing the calculations on tensions that are based, for example, on ISO 5048 static type calculations, which in the real world are about as accurate as me trying to play darts..

You have to know when dynamics plays a part. For this and other reasons I wrote the paper on "Golden Rules" for conveyor design which was persented at Belcon 13.

In this paper I demonstrated via case studies, pulleys (especially LT ones) have been known to fail due to dynamics being ignored.

I also indicated that if you can't even spell dynamics, what precautionary factors can be used.

Regards

LSL Tekpro ■

As conveyor pulley designers we have to assume (and we all know what that stands for) that the conveyor designers have done there job correctly and what figures we are given to design with are correct. If the conveyor designers give us wrong numbers then we calculate a wrong shaft size – simple. With respect to dynamic or static calculations for determining belt tensions I leave that upto more skilled people than I to determine. I am aware that there are a number of commercial software programs out that do allow for said dynamic effects.

I agree that more often than not when there is a shaft failure and a root cause analysis is properly undertaken then often it is the overlooked dynamic effects that are the cause. ■

Its not just pulleys that suffer from mis-information either.

I presented another paper at the "Loading Conference" in Johannesburg, which was organised by our civil/structural engineering counterparts.

The conference examined the risks involved during the transfer of information from discipline to discipline and what load cases civil/structural engineerings should ask for.

Here they got to realise why they had lost conveyor structures as their mechanical counterparts had for example, never even heard of the expression "aborted start" or anything else dynamic for that matter, and they normally get bum information to work from.

Too many overnight experts in this field I fear..

LSL Tekpro ■

Many a true word ........ ■

Hi,

I looked at the shaft design comments above. I am currently doing an assignment for uni where we have to design an intermediate shaft (situated between a belt drive and chain drive) which is part of an air compressor drive system. I hope that makes sense. Basically, we have been shown to follow the way to design the shaft as was mentioned above, calculating loads, calculating bending moments, torsion, choosing material on that, etc, etc. Then someone only mentioned ISO keyway. I'm now up to the part in my assignment where I am to determine the required dimensions for appropriate keys and keyways in the intermediate shaft for the large pulley and small sprocket. I'm quite clueless. Can any one help with how to design keys and keyways?

I also need to design a suitable bearing pedestal (the shaft has two deep groove ball bearings on it) that is to be made by sand casting. I'm also quite clueless here. I then also have to do a general assembly drawing of the entire pedestal assembly including the shaft, bearings, seals, etc together with "catalogue" dimensions and a material/component list. (you may now understand why I am so sick of this assignment, it never ends).

If anyone could help with comments, or websites, or where I can find useful information about what I'm mentioned, that would be greatly appreciated.

Thank you! ■

Hi,

I looked at the shaft design comments above. I am currently doing an assignment for uni where we have to design an intermediate shaft (situated between a belt drive and chain drive) which is part of an air compressor drive system. I hope that makes sense. Basically, we have been shown to follow the way to design the shaft as was mentioned above, calculating loads, calculating bending moments, torsion, choosing material on that, etc, etc. Then someone only mentioned ISO keyway. I'm now up to the part in my assignment where I am to determine the required dimensions for appropriate keys and keyways in the intermediate shaft for the large pulley and small sprocket. I'm quite clueless. Can any one help with how to design keys and keyways?

I also need to design a suitable bearing pedestal (the shaft has two deep groove ball bearings on it) that is to be made by sand casting. I'm also quite clueless here. I then also have to do a general assembly drawing of the entire pedestal assembly including the shaft, bearings, seals, etc together with "catalogue" dimensions and a material/component list. (you may now understand why I am so sick of this assignment, it never ends).

If anyone could help with comments, or websites, or where I can find useful information about what I'm mentioned, that would be greatly appreciated.

Thank you! ■

The size (width) of a standard keyway is based on the diameter of the shaft. You need to refer to the appropriate standard to find out what is the size for your shaft.

Then you have to work out what length you need taking into account the shear strength of the key material and the bearing pressures between the key and shaft. Of course you may find that the length is dictated by the pulley and sprocket you are using.

"I also need to design a suitable bearing pedestal". Why?? SKF, FAG et al make them by the thousand and you won't beat them on price. If you must make a special use one of these as a guide.

"the shaft has two deep groove ball bearings on it". In my view not a good idea, use self aligning bearings as they can tolerate a bit of misalignment on assembly.

"I am so sick of this assignment". Trust me, this assignment is easy, life is a lot harder in the outside world!!

PS, not usually regarded as good form to cross post in different forums. ■

Hi,

Thank you for your reply designer.

Hopefully some of that information will come in handy.

I am sorry that I posted the message twice, but I am not aware of how quickly people check this site and respond and I thought I had more chance of someone seeing the message and maybe responding if I posted it under a more appropriate name (rather than the one I originally posted it under). Thank you and I will not double post a message again. ■