How much Draw for Interference fit on a Taper

[not2fast]

I've had several idea's about how to secure a small Supercharger rotor to the shaft that I'm building. Rather than a conventional press fit & parallel shaft I was thinking to use a Taper of 1 degree, but I'm open to suggestions.

The Rotors are 6061 Al, OD 75mm, Length 150mm.
The Shaft is 4140, the larger end of the Taper is 18mm and 15.38mm and 150mm long, each shaft end has 15mm dia shaft for the bearings and gears.

one idea thats kinda neat used on MTU engines is the use of a Taper (approx 50:1) and a High press pump is used to expand the Flywheel while a 2nd Hyd ram is used to push the flywheel into position and then release the pressure.

I think that something like this method applied to my Rotor and Shaft setup would be ok, as it will allow the rotor to be positioned on the shaft so that clearance's with the adjacent rotor and axial clearance with end plate's can be adjusted... is this to complicated perhaps ?

My question is how much "Draw" or oversize should I make the shaft to securely hold things in place without failure.

Appreciate any thoughts.

not2fast

 

[eromlignod]

The amount of interference of your fit will depend on how far you drive it onto the shaft. Your shaft and hole need to both be tapered and your interference will be the difference between the large end diameter of the hole and the diameter where this end ends up when you stop pushing. How much interference you need depends on what your application is. You can look it up in a standard fits table.

18mm might be a little small to be boring a taper, so you might want to use a tapered reamer. You probably won't find one that's exactly one degree, but for a permanent fit you could use a taper-pin reamer. Taper pins are a 1:48 taper, or about 1.2 degrees. A reamer for a #10 pin should do it.

Beware when you are pressing this though. It is not going to slip right on smoothly to your stop point. As you wedge it tighter and tighter it will start popping and squeaking and jumping, so you might miss your mark a little. Also, be sure that you don't make your interference too much for the length, so that the shaft won't start into the large end of the hole!

Don
Kansas City

 

[IceStationZebra]

Such a slim taper does not make a precision positioning feature. If you are going to make more than one or two of these you should find a different way to hold the rotor in position.

I would also be concerned about heat expansion, especially with dissimilar metals. The rotor is going to heat first and expand much more than the shaft.
6061 23.6 x 10-6/°C
4140 12.3×10-6/°C

And don't forget that the dissimilar metals will tend to corrode together making disassembly a pain.

ISZ

 

[hydtools]

Morse taper for machine tools is about 1.5 degrees.

But like ISZ notes, change in size with temperature could be a problem.

Ted

 

[not2fast]

Appreciate the replies,
So what would you all suggest to secure a shaft & Rotor ?

maybe just a Parallel shaft with 2 Tapered pins locking the shaft and rotor is more practical perhaps ??

I contacted the Guy who machines my parts he informs me that he can Wire cut a Taper inside the Rotors, whats the downside to this method apart from the small raised section where the Wire enters and leaves the cut ? Lapping both parts would remove any raised material.

A shoulder on the shaft would provide a positive stop for the Rotor to press up against would work ok, and adjust axial clearance with shims...not such a big deal.

I have also given thought to heat and expansion of the Rotor resulting in less interference between the shaft and rotor, so if max temp of the Rotor was approx 100degC (hotter than I would have expected) would I still be safe enough with an interference of 0.05mm to 0.075mm (0.002" to 0.003") and is this going to added to the 75mm OD ? so will I now have a Rotor OD of 75.05mm ??

The Initial install will be a Carby so the A/F mix should keep the temps reasonably safe... a future plan is EFI.

not2fast

 

[hydtools]

Use a reamer to finish the tapered socket. With positional control of the tapers you can depend on the taper to position the rotor on the shaft. Use a center bolt to pull the rotor up on the shaft. Chose a gage diameter on the taper and specifiy its location to a datum on both the shaft and rotor.

That is what I did to connect a single bearing alternator to a hydraulic motor to create a solid shaft connection between the motor and alternator rotor.

Ted

 

[Warpspeed]

I might be tempted to adapt the taper from a commercial taper lock fitting, and key the parallel shaft in the normal way.

If using a conventional taper, I think I would shrink the pulley onto the shaft rather than press it on. Or at least press it on hot.

 

[IceStationZebra]

I have installed large bronze bushings by putting them into liquid nitrogen first. Pretty cool

My first idea was to have some serrations on the shaft that interfer with the rotor. You wouldn't be able to take it apart again - but it is simple and should be able to tolerate some thermal expansion.

I also ran across these images. One is from an Eaton M90, not sure about the other. It appears that the rotor shaft fits inside the drive shaft, and the torque is transferred with a dowl pin. Not very elegant but you don't have to worry about spinning the rotor on the shaft. (threaded end is for the drive pulley)

ISZ

http://img136.ihttp://www.dkimages....geshack.us/img136/5172/supercharger031cy4.jpg

ISZ

 

[Tmoose]

The inner ring of Precision ball bearings in high speed spindles will tend to grow at high rpm. It can be 0.0005 inch diametral, maybe more. The initial fit on the shaft must be that much tighter to preclude loosening at high rpm.

I'd do a little modeling and figuring to predict what a supercharger impellar might do.

 

[JamesBarlow]

My first thought would have been a simple parallel fit. Heat the rotor and freeze the shaft.

I would also include a key to ensure that you don't spin the rotor. My understanding of superchargers is a little weak but I'm pretty sure they don't run that high a pressure to avoid overheating the air but I would key the everything just to be safe. A spun rotor will send pieces of metal into you engine which will cost a lot more then putting a key in.

A key will also aid in final assembly. Put keys on your gears as well and you can get everything to synch up nicely without needing to do any post-assembly machining.

Given the size of your rotors how much power increase do you think you will see? The only blowers I have ever seen apart are for large block hot rods and the rotors are pretty big on them.

 

[MFredriksen]

Why not go with a standard tapered fitting? Key the shaft and adapter. What kind of rpm are you shooting for?

 

[not2fast]

Thanks for all the replies,
My thought's about the parallel shaft, is this setup more difficult to obtain a good fit between the two parts, I mean actual contact between the shaft and the Rotor in this case thats 150mm of parallel shaft. IIRC some figures for a good machined surface was only around 40-50% of actual contact maybe this was a Loctite promotion hype.

What I like about a taper at say 50:1 is the ease of (dis)assembly, and depending on the amount of interference this will control how many mm of draw or "push-on" for the final assembly. I'll try some test's with a wire cut taper and a ground shaft lapped in and see how strong it is.
If I applied some heat @100*C or 150*C to the test Rotor and apply a test Torque of XX ftlb's would this be any indicator of whats to come ? now what about the centrifugal force's at play @ 10K RPM ?
I still plan to assemble these parts with High Press oil injected between the Shaft and Rotor, I have also madded a shoulder for the Rotor to seat against, perhaps a Tapered pin is needed.

RPM's of the Blower will be approx 12,000 max, according to my quick and dirty calc's 15HP should be close for power req'd for the blower, say its 20HP for some extra safety, thats approx 10ftLb's @ 10,000RPM (Engine) So my understanding here would be that each Shaft-Rotor contact only needs to be able to withstand 5ftlb's @10,000RPM being as there are 2 shafts sharing the load.

Perhaps a failure here and there will guide me LOL

BTW how much would a 16mm hole expand if heated to 100*C or even 150*C for 6061 ?

6061 23.6 x 10-6/°C
4140 12.3×10-6/°C


not2fast