Shafting material and heat treat question

[rjwoods77]

I have a shaft that I would like to run on an HJ series needle roller bearing without the inner ring. After running the M-V diagrams and such with safety factors I have a max stress of 85,000-100,00 psi. When I was in formula we ended up running a 4340 shaft heat treated and normalized to 48HRC. In order to run directly on the needle rolled I would have to have an 58HRC and a surface finish better of 16. Case hardening and grinding will yield these results but I am unfamiliar of what material I could use that is appropriate for the stress reversals and material strength etc. Any help would be appreciated.

[VFR750R]

Depending on your application, a 'tool' steel might be what you are looking for. To make a long heavy shaft out of tool steel can get expensive so I can't recommend a solution without knowing more about the application. But if you look up on matweb.com or in a materials book stuff like S7, D2, A2, etc. it might give you some ideas. Most needle bearings are made of 52100 bearing steel, so it will have to be compatible with that. I think you'll like S7, it has a HRc of 59-61, and has high impact and shock resistance. Yield of 220,000 and ultimate of 293,000psi.

be careful of case hardening as it typically isn't very thick, and you can grind it too thin or right off if your initial parts aren't close enough to finished size before case hardening. You should also look into nitriding (which works best with high chrome content steels).

[rjwoods77]

Its for a jackshaft on a mini-bajaish car. The shaft will have two bearings a cvt and a primary sprocket. Page 3622 of mcmaster.com has it is a soft condition and the prices are comparable to a 4340 bar 1.25 OD is about 30 bucks a foot. There are some other options on page 3611 in terms of alloy steels that would get high enough such as 8620,4150 and E52100. WE used 4340 at HRC48 because of the Caroll Smith recommendation. We never really stopped to take a look at anything else really since I/we didnt know much past that. The plan is to get the barstock in an soft condition (annealed) cut it oversize, gun drill it and then heat treat with final grinding on OD's and stoffel splines.

[VFR750R]

Its really hard to beat 4340 for what it'll do and the price. S7 might work, but I hate to recommend it and it be a problem.

Sounds like this is more like a budget build anyway, so making 4340 work might be the way to go. Of course you'll be heat treating but you might also consider cyrogenically treating. It finishes the conversion of austenite to martinsite, giving a higher tensile strength and harder material. It also promotes carbon precipitation which increases wear resistance (some claim 200-400%). Might be enough. I'd try to find out what transmission manufacturers use for shaft materials for some more examples.

Have you thought about a bushing instead of a roller bearing? Brush wellman makes some bad ass bushing material called tough-met that would drastically lower your peak loads compared to a roller, and has really high PV ratings. It might hold up to dirt and grime better too. You're going to need seals with those roller bearings.

[rjwoods77]

The bearing load is around 5000lbs peak load at rpms as high as 3600rpm so a bushing wont do it nor would I feel comfortable having one there. The needle bearing is the right size and it has double seals on them so dirt and crud aren't a concern. The reason I want to run it right on the shaft is because if i use a inner ring then I have to retain it axially from moving which means a shoulder on one side and I have to run a snap ring groove on the other side of the bearing.

All the materials look to be the same price so it really doesn't matter. Jackshaft is only 14 inches long. I don't really care what it is made out of as long as it has 4340 torque reversal traits while having an HRC58.

[Drew Price]

Did you consider a bearing with a press-fitted inner race?

Or you could use press-fit collars to make up the shoulders to locate the bearings, or you could use slip fit collars with set screws and grind little flats on the shaft for them to tighten against so you don't have to cut grooves or keyways in the shaft?

Rob, I don't know where you ended up, so I also don't know what machine access you have, just throwing ideas out.

Best,
Drew

[VFR750R]

So how are you limiting axial movement of your shaft if the bearing is not controlling it?

[rjwoods77]

There really arent any axial loads to contend with it just needs to be axially restrained. For space/load reasons I am trying to do a needle bearing next one one end and the other is just a normal ball bearing. The shaft will have a shoulder that butts up against the inner race on once side and a retention nut on the other side so that will take care of the axial location. Keeping an inner ring from floating is the challenge. I was thinking today that a shoulder on the shaft next to the inside of the roller bearing and a sleeve that rides over the shaft between the bearing and the sprocket might do the trick. Again I rather just run the shaft straight on the bearing and be done with it. Maybe a nitride or DLC coating will do the trick. Gotta start calling heat treat places and talk to the local know-it-all. Then again if I can find shaft that takes load reversals well that I can harden then all the better. I was looking in the machinists handbook today and 1095 can be brought to 60HRC while being very strong. Page 434 of the 26th edition handbook. Table 7b. Gotta start calling the people who know this shit. Might call a transaxle company since they use needle bearings a lot. Scotty from Taylor Race might have somebody he knows. I appreciate the input and will look into what you offered. Any additional would be welcomed.

[dazz]

My 2c is that you should design it as a press on inner race as Drew has suggested (or something to the same effect). Seeing as needle rollers transmit SFA axial loads, a light press fit inner race should stay in position fine, a bit of bearing retention compound can be used if you're really worried about it. Reason being that when the bearing wears out or if a bearing failure occurs, your custom machined & hardened shaft is stuffed, requiring a costly & time consuming replacement to be manufactured.

[slingracer 62]

I also agree with the inner race if this is possibe for you to design around. My team has used needle bearings before on 4XXX series material and it is hard to get anywhere near the surface hardness to get any wear out of it. I would stay away from any 10 series as you can get that hardness but you will be brittle like none other. A jack shaft will see shock loading so I would think you'd want some ductility and temper at atlest 450 F or higher. Just remember not to try and pick 550 F because temper embrittlement can be an issue. Look towards tool steels or the most proper solution would be a surface treatment such as a carberized case which a softer core would still give you strength and ductility requirements

[Wrong Way Sam]

I think it would be overkill to use a tool steel or anything that his high carbon. The 4340 would be fine if you could get a 0.020 or 0.030 case depth (depends on bearing size). Without knowing much about the layout you would only have to grind a portion of the jackshaft to where the bearing located. With use there is the potential for damage, but you could always turn the case down and press a sleeve on later. If your jackshaft size is small enough that you would be concerned about shaft life I think you are designing a little to close to the limit. I would stay away from any coating or nitriding in this case as you probably don't have a lot of time or money to experiment with heat treat processes.

[rjwoods77]

Here is what the setup looks like. There will be a stoffel spline on the end for the sprocket and it represented by the step. Threaded ends on both sides for retention nuts. You can see that the shoulder on the inboard side and nut combo takes care of axial location. This shows a shaft with no inner ring for the roller bearing. I cant do a pressed on inner ring since I have to be able to slide the secondary cvt on to the shaft so I would have to have some sort of axial location on the ring.

I guess if I redesigned the mechanism this fits into from a pivot bearing to a pivot bushing then it would solve my od restriction, put 2 2"odx1"id bearings out at the end and solve all of this all together. I love how one thing connects to the next connects to the next.

Wrong Way Sam,

I was looking through the engineering section of the Timken catalog and found that maybe I need to not use a needle bearing there. The maximum slope for the bearing as described in the book says no more than 0.0015 and I will be exceeding that with shock loads. Looks like my OD restriction for the bearing will have to loosen up.

http://s582.photobucket.com/al...urrent=jackshaft.jpg (http://s582.photobucket.com/albums/ss261/rjwoods77new/?action=view&current=jackshaft.jpg)

[Superfast Matt McCoy]

"mini-bajaish car."?

I move to add an alumni projects section to the forum.

[flavorPacket]

300M can get you 58 HRC. Great fracture toughness too.