Coupling

[shipdesigner]

I'm designing the coupling for a 63.5mm diameter propeller shaft but I don't know how to check if it's strong enough to hold the shaft. Any reference, calcs, info, links and suggestions, so I could check it? thanks.

[CDK]

You'll find the answer at mitcalc.com , assuming that you choose one of the conventional ways to lock the flange to the shaft, like splines or keys.

[shipdesigner]

thanks. but this site has no specific calc for the said coupling.

[CDK]

I understand that you want to rely on the conical joint to take up the torque. In mechanical engineering that is considered to be a doubtful construction because much depends on the surface quality and cleanliness at assembly time. If for any reason, a key is no option, I would use a less steep cone and a spring washer under the nut to compensate for thermal changes.

[Frosty]

What Hp are you putting through it.

If you stick to the standard shaft /prop/length/ HP/calcs readily available and make the standard sae taper you should be well OK.

Taper and key is the norm and has been for generations in all applications. If kept tight this should be sufficient.

Ive never seen a prop or flange strip its key,!!! come loose yes, but strip no.

The taper and key of the shaft is often made the same so the shaft is reversable. This is so any wear from the stuffing box on the shaft can be overcome by reversal.

[shipdesigner]

CDK, thanks. Frosty, thanks. That word SAE means so much to me.

[Frosty]

http://www.australpropeller.com.au/machining.htm

[shipdesigner]

Thanks to all who responded. It was very useful.

I have modified the design and I come up with the coupling design below.

There are four (4) items which I'm not sure:

1. dia 110 (coupling dia)
2. 46 (coupling hole depth)
3. dia 80 (coupling hole dia for locking nut)
4. locking nut dimension (depth & dia)

Could someone please shed some light or givea formula, rule of the thumb or any reference. Thanks.

[jehardiman]

First of all, you need to answer a couple of questions.

1) Where is the thrust block? Have you accomidated thermal expansion?
2) How do you intend to press the coupling on? Pilgrim nut?
3) A single locking nut is insufficient without some form of keeper, how are you going to keep it locked? (Besides how are you going to tighten the lock nut...custom pin spanner?

Generally, tapers are sized based upon the thermal and bulk moduli of the two materials to be jointed. Most still require a key to prevent slipping during installation and generate maximum effeciency. Check out the Machinery's Handbook for SAE shaft tapers (steel/cast iron) of the size you are looking for. For other materials, you need to go back to first principals and calculate the radial stress based upon installation axial load, work up the residual torque, size the keyway to provide the required FOS then back check the stress risers in the shaft/coupling.

For a nominal SAE 2 1/2" shaft. Taper length is 3 1/2", taper is the standard 1.5000" per foot, coupling OD is 4", square 0.6250+0.0015/-0.0000 key.

BTW, it is incorrect to dimension the small end of the taper. Dimension the large end and call out the taper. Then cut a stress reducer at the end of the taper where it joins the threads and dimension the threads as normal. Will save you a lot of headaches later.

[CDK]

The thread will of course be something like M50x2 (Metric fine). Nuts that size are very hard to find but can easily be made, in which case you can specify any dimension you want.
With a hole dia of 80 there is no room for a hex nut, only a cylindrical one with 4 holes, for which a special tool must be made. There are ISO standards, but deviations for anything above M39 are quite common. The nut in your drawing looks OK to me.
Overall size is dictated by torque requirements, but you did not specify them.

[shipdesigner]

Originally Posted by CDK

...Overall size is dictated by torque requirements, but you did not specify them.

Maximum torque is 3,000 N.m.

[lazeyjack]

well it look like there is not room for your screw heads(or bolt heads) 7/16 un has head of 11/16 or 17 mm A 2 1/2 in shaft would need better than 7/16 unf thread size

[jehardiman]

Originally Posted by lazeyjack

well it look like there is not room for your screw heads(or bolt heads) 7/16 un has head of 11/16 or 17 mm A 2 1/2 in shaft would need better than 7/16 unf thread size

Yeah, I didn't notice that. Width across the corners of a M16 bolt/nut is 27.7mm. With a bolt circle dia of 140mm and a coupling OD of 110mm and a fillet radius of 5mm, the bolt/nut can't be turned. (Besides earning you the hatred of everyone who ever works on this coupling because they can't use a boxed end on it.... ). You need at least 16-17mm from the toe of the fillet to the CL of the bolt circle.

[Frosty]

I cant see the picture of you drawing too well but dont forget an allignment ring to engage the other flange. Don't rely on the bolts for centralization.

If the bolt heads are too wide you could slice a bit off the head of the bolt and have them fit close the the flange body.

[lazeyjack]

Originally Posted by Frosty

I cant see the picture of you drawing too well but dont forget an allignment ring to engage the other flange. Don't rely on the bolts for centralization.

If the bolt heads are too wide you could slice a bit off the head of the bolt and have them fit close the the flange body.

you sure you are a butcher by trade Frosty? Alignment is by clock dial or at least feelers and straight edge You force two halves together you end up with chattering gear teeth and vibrating engine