shaft connections, high torque, no backlash

[bdeuell]

I am designing shaft that will transmit torque from a pulley at one end to a plate near the middle. The shaft is roughly 4” long and I am hoping for a .5” diameter shaft. I might be able to go up to a .625” shaft without causing catastrophic problems elsewhere in the assembly but any bigger and I am not sure I will have the room. I was planning to use 4140 for the shaft and heat treat but this material election is flexible. The plate is .5” thick will likely be made from the same material. The pulley is aluminum and approximately .750” wide. The shaft assembly will need to transmit a torque of 1300 in-lb through a rotation of 120 degrees at low rpms. There needs to be very little or ideally no backlash.

I am looking for suggestions on how to make the connections to this shaft for both the pulley and the plate. Currently I have shaft keys in my design but I am concerned about the backlash so I am looking for alternatives.

I have looked at using a tapered locking hub (see Link) but it would require at least a .75” shaft to even get close to the required torque rating.

I have considered a tapered connection directly between the shaft and the pulley/plate but this would require using a larger shaft as the pulley comes with a .5” bore already drilled. If this is the way to go I could bolt a new hub to the pulley or if necessary I may be able to go to a larger shaft size.

I am concerned about designing my own friction based (tapered) shaft connection without prototype testing. As this is a one off production I can’t really afford to test multiple ideas in search of a solution. Of course I expect to do some prototype development during the build but I want to be sure I am on a design path that will deliver a solution. Surely someone has done testing on the torque transmission of tapered shaft connections. Does anyone know of a source for data that could be use to design a tapered shaft connection?

Another thought was to use a flange plate to transmit the torque. This could be welded on or machined out of a large piece of stock (neither sounds very appealing to me but they are options). However, this would only work for the plate in the center of the shaft as the pulley must be removable for assembly/disassembly.

I would appreciate any help on this problem.

 

[GregLocock]

What would be the limit on the length of a taper, which gut feel suggests is the only likely solution on such a small shaft?

I certainly wouldn't expect any nonsense with keys to work. A spline might, if you could roll it.


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[CoryPad]

Any restrictions on maintenance or part disassembly or replacement?

Splines are a common method for these joint types. Rolling, milling, broaching, and axial forming are all methods to produce splines.

Bolted joints also are used widely for shaft/pulley connections. A spring pin or retaining ring may be options for the plate connection.

Can you share a picture of the design?

 

[drawoh]

bdeuell,

How acceptable is it for parts of your drive train to be flexible?

--
JHG

 

[bdeuell]

Thanks, for the responses.

The taper length is limited to roughly the width of the component being attached, .5" for the plate and .75" for the pulley.

As for splines I assumed they would have backlash, is this true?

The plate can be permanently attached to the shaft but the pulley must be removable.

I have attached a screenshot of the design with some notes added.

@drawoh
What do you mean by "parts of your drive train to be flexible"?
The design uses a synchronous belt drive train so I expect there will be some amount of spring to the drive. I do want to keep the drive as stiff as possible but some small degree of spring is acceptable.

 

[MintJulep]

Study the square-taper connection used on a bicycle crank.

 

[MintJulep]

http://sheldonbrown.com/bbtaper.html

 

[MikeHalloran]

Multifunction shafts like that are normally stepped, for ease of manufacture and for ease of assembly and for bending strength.

Splines only need backlash if they have to slide.
In this case, a tapered clamping hub as you have suggested can probably work well, if its 'bore' is not a simple smooth bore, but an internal spline, and of course the distal end of the shaft needs to have a mating external spline that's a slip fit until the clamp is tightened.

The splines and other features will be stronger and easier to manufacture if the steps in the shaft are not sharp steps, but tapers or ogival transitions, which requires narrowing the elements or spacing them out a bit.



Mike Halloran
Pembroke Pines, FL, USA

 

[TVP]

Polygon shaped shafts are another way to improve torque transmission. Here is one source of information on the subject:

http://www.stoffelpolygon.com/index.php?option=com_content&view=article&id=4&Itemid=102

 

[BrianE22]

Ringfeders (

http://www.ringfeder.com)

are the best I've seen for transmitting large torques. The smallest they seem to go is a 3/4" shaft. Maybe you could custom make one for your size shaft.

 

[bdeuell]

I appreciate all of your responses.

After pondering your suggestions this weekend I think I have decided on a design.

For the pulley I am going to use a tapered sleeve with a triangular profile inside. The triangular profile will engage a matching geometry on the shaft providing a positive engagement. The sleeve will be drawn into the pulley with a matching taper by a few bolts, compressing the triangular profile ensuring a tight fit. I am avoiding a spline connection because I do not have the tooling to produce one in house. I opted for the tapered sleeve over the tapered polygon/square in this application as it will allow the use of jack out screws for dis-assembly but that was a strong consideration.

For the connection with the plate I am thinking I will do a shrink fit taper. I am hoping with a shrink fit and pressing the taper together I will be able to transmit the torque using a standard conical profile on the taper. Does anyone have a good source/reference for designing a tapered shrink fit connection, particularly calculating the torque rating?

 

[desertfox]

Hi

Go to page 27 on this link, it gives torque formulas for tapered connections

http://turbolab.tamu.edu/proc/turboproc/T30/t30pg121.pdf