Tapered Splines 3

[MaxB89]

Hi all. New user to the forum. I am designing an Anti Roll Bar for a train. Customer has requested that we use a tapered spline. Not something my company has experience of and there don't appear to be any ISO or other standards on the subject. Has anyone got any ideas where I can find some information or designs for theses? Otherwise I am starting from scratch which I really don't have the time for.
Thank you in advance....

 

[SwinnyGG]

When they say 'tapered spline' do they mean something like a 'keystone' spline, where the shaft is cylindrical but the tooth width (for male) and opening width (for female) taper along the cylindrical axis, or do they mean that the faces on which the splines are cut are conical?

It is also possible to combine both- conical splines which taper in width along the axis of the cone.

In short, there is no standard for either; cylindrical splines with keystone ends exist and are common in some transmission shafting, but are not common enough to be inexpensive.

In general, a tapered spline is not a very good idea; case 1 and case 2 are both significantly weaker than a simple cylindrical spline would be, with the added issue that without very precise component positioning, they will have very large amounts of backlash.

The combination of case 1 and 2- where the splines are conical AND tapered along their length, is even weaker still.

This is one of those times where in my opinion, you should be recommending to the customer that they change their mind about what they want.

 

[MaxB89]

Thank you for your reply jgKRI. The version they are after is case 2. Splines along a Conical surface.
Believe me I have tried to convince them that it is not the best solution but unfortunately they are adamant this is what they want. It is worth point out that another manufacturer currently makes this for them and they are looking to change to my company so they know it works. And of course they cannot share the design information with us....
I obviously don't want to say who the customer is but they are a very large, well know German company. Not the sort of people who will listen when you tell them there is a better way for doing things.

From my understanding the reason behind using a tapered spline in this instance is for ease of removal for servicing etc.

 

[SwinnyGG]

Fair enough. We've all been there.

There's only so many very large companies who manufacture rail equipment... so I'm fairly sure I know who you're talking about.

External straight splines on a conical surface are easier to produce than tapered splines on a cylindrical surface, since you can use existing hobbs/broaches to produce the forms; the internal conical splines will be a challenge.

If I were in your shoes, my first move would be to speak with my machinists, whether that's an internal or contract resource, and determine if they've done this before, and if they have any recommendations to guide your design work. If everyone in your supplier portfolio says they've never done it, I would keep them in the loop during the design phase so that your final design is as easy to manufacture as possible. This may not be the type of part that you can just design and send to everyone in your supplier portfolio for RFQ.

 

[chez311]

MaxB89,

Out of curiosity how exactly do these splines assemble? When assembled is there clearance between the male/female splines (some other feature stops them before they wedge together in interference, say a mating vertical face or not assembled with any preload) or is there interference (ie: male/female splines are wedged together with some sort of preload)?

 

[SwinnyGG]

I was envisioning this as a cup-and-cone sort of arrangement, where the cup has a thru hole and the end of the cone is threaded for a fastener. Similar to many tapered coupling arrangements, with splines instead of smooth faying surfaces.

Of course only Max can say if that's remotely correct. I'd be curious to find out what it actually looks like.

The fact that this is a railcar component also means the sizes of these parts could be quite large.

 

[GregLocock]

It's almost like an internal and external straight cut bevel gear pair with the same number of teeth on each part, with a very high bevel angle (if that's what it is called).

Anyway SAE J501 table 2 describes a thing called a tapered shaft end with (parallel sided) key. Light reading on the subject reveals that SAE calls some types of splines serrations.





Cheers

Greg Locock


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[MaxB89]

jgKRI is on the money. The "cone" in on the end of the bar. The "cup" is in the lever arm with a through hole and threads on the end of the bar for a nut to clamp it down.

Chez, no there is no vertical face, the nut tightens the 2 tapers together in a wedge. We do this arrangement a lot on ARB's. Just not usually with splines. Or we do splines without the taper!! Or the normal square drive, even circular shrink fits. Just never tapered splines.

 

[chez311]

MaxB89,
Thats interesting - I've mused a few times about whether this would work on certain applications but I figured that wedging conical splines together like that would run the risk of creating a sort of press-fit between the splines and cause issues with service/removal. Apparently thats not the case (as you said its for ease of service) - its probably highly dependent upon taper angle and the design of the splines themselves/tooth form. Or maybe thats a non-issue almost regardless of design - I've never actually seen this put into practice. If you are able to get any details about the current design/spline specification, no matter how approximate, is there any chance you could share them in some capacity? I understand that might not be possible due to a variety of reasons I'm just very interested in how they made this work.

jgKRI,
Do you have any examples of a similar design? (either keystone or conical splines) Like I said - I'm very curious about whether a very specific spline design is needed to make this work or if its not as sensitive as I'm thinking. Also you mentioned that these types of designs can have a significant amount of backlash - how is that exactly? I would think that assembling the splined connection by wedging them together would eliminate any relative movement.

 

[SwinnyGG]

Keystone splines are common in dog transmissions where the dog teeth need to not slip off the end of the splines; the keystone shape is used to provide some backward force on the dog ring and pull it onto the shaft.

I know of no readily-googleable examples of conical splines- but these splines are backlash prone because they have to have some sort of axial locating feature- as in OP's design, the cup seats against the end of the cone and is tightened by a nut.

For a normal spline arrangement, you have to worry about the shape and tolerances of the two spline sections, which is already a very precise machining operation that needs to be right. These interfaces, however, have a benefit- axial positions of the components relative to each other are not very important; this removes a degree of freedom that must be controlled during manufacturing.

With a conical spline, you have to worry about the form/tolerances of the splines, AND the form/tolerances/relative positions of the locating features too. If any of these are wrong, the set either won't mate at all, or will mate with excessive backlash.

 

[chez311]

jgKRI,

Sorry to be thick - I hate to ask but is there any way you could sketch up a quick diagram of what you mean? I'm having trouble visualizing what you're talking about, I initially imagined that these splines are directly wedged together which would accomplish all axial and radial locating by the spline teeth alone (ie: on the pitch diameter/cone) and "should" pretty much remove all possible backlash as there would be no clearance between the splines.

 

[SwinnyGG]

I'm not in a place where I can draw a diagram really, but I know what you are asking.

What you are describing is one possible way to fit the splines- but it's also the one that doesn't provide precise axial location. Axial location of the parts in this scenario would be determined by the tolerances of the two sets of splines and the force applied to the coupling fastener holding them together, which will not be all that precise. It might be precise enough for the job, but it's not an easy part to make.

 

[MikeHalloran]

I swear that I have disassembled a worm-and-sector steering box that had a rolled tapered spline on the output shaft.
Getting the steering arm off required using a pickle fork after loosening the nut, but was otherwise unremarkable.
I have no idea how they made the internal tapered spline in the big end of the steering arm.



Mike Halloran
Pembroke Pines, FL, USA

 

[gruntguru]

Link

je suis charlie

 

[MaxB89]

Chez, if you imagine the problems you have with a normal tapered joint, you have you design the tolerances so that the tightest fit is at the major diameter. So your external has to have a tolerance of +x/-0 on the angle. And your internal has to have a tolerance of +0/-x. And this tolerance is already in minutes and seconds.
Then of course you have to make sure the axial placement is correct and the clamp load is right in this position. Once you add splines into the equation you no longer have 1 clamping surface but hundreds. Very fine tolerances and almost impossible calculations to work them all out. Hence why I am hoping there is a standard somewhere that has done it before. I have been put on to 2 old ZF gearboxes standards that may help but obviously no access to them.
Thanks for the link guru. That may be worth buying just to see what's going on in there.

 

[GregLocock]

Do you broach the female part?

Cheers

Greg Locock


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[MikeHalloran]

I can't imagine how to broach the female tapered spline.
I can see doing it on a gear shaper, but there must be a much faster way.


Mike Halloran
Pembroke Pines, FL, USA

 

[chez311]

jgKRI,
I agree - relying on solely on the spline geometry for all radial/axial location and angular alignment could be problematic especially when combined with possible variations in clamp load. I wonder if designing a conical splined joint with a shallow taper combined with a relatively precise flat axial mating face would be possible - this way the splines would engage first and as the male/female splines are wedged together and elastically deform slightly the axial feature is then engaged and locates the assembly axially. Think of something like dual-contact (BIG-PLUS Daishowa trademark) tapered toolholders for CNC machines.**

MaxB89,
I just want to confirm, because you seemed to agree with jgKRI but he also seemed to have something different in mind, would the splines in your proposed design would be directly wedged/clamped together accomplishing ALL radial/axial/angular location and alignment? Your comment makes me think that this is the case because you're taking about clamping surfaces directly on the spline teeth. Maybe a diagram would be helpful - sorry about my confusion if I'm missing something.

MikeHalloran,
I'm not extremely familiar with the technology but perhaps depending on the relative size of the components involved (may be problematic with very large or small components) power skiving would be an option? Seems like this method could definitely handle an internal conical spline.

https://www.youtube.com/watch?v=qsDowA8-sp0

**Edit: I should say obviously the difference being that instead of a ground conical taper there would be tapered splines but the dual contact concept is similar.

 

[BUGGAR]

I can't figure out how this could be machined but I believe I have seen something close, in a short version, on water jet cutting demonstrations.

 

[GregLocock]

An old fashioned shaper could do it. Pretty much bang a cold chisel into the workpiece a few thousand times.

Cheers

Greg Locock


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