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Case Hardening vs Through Hardening

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Plastic
Joined
Apr 5, 2017
Which style of hardening is appropriate for my application? (see below)

I'm making a pawl for the kickstarter shaft of a 1960's Riverside/Benelli motorcycle. The pawl goes from the kickstarter shaft to a gear on the engine, to spin the engine for starting. The orig. design was poor, and these pawls wear out prematurely. The engine gear has many teeth, and the single tooth of the pawl wears fast. This pawl is buried deep inside the engine, so I'd rather not replace it very often.

The factory supply of pawls was exhausted years ago, so I'm making one with a dremel and hand tools. A diagram of the pawl is attached.

To keep the new pawl from wearing out, should I use mild steel and case harden it, or use tool steel and through harden it? Keep in mind that I am a home hobbiest, with limited resources.

-Part Number H8097 Kickstart Key.jpg
 
first off find out what type of material BENNELI used back then...it was more than likely grade 5 or grade 8 regular steel...non hardened....but look at their spec sheets or prints....I do this with locomotives like rebuilding a SHAY LOGGING LOCOMOTIVE....its all right there of what they used...how they heat treated if necessary and so on....I would think bennelis are online too
 
How often do you want to replace the engine gear?

How often do I want to replace the engine gear? Never. Removing/replacing transmission shafts to get at the gear, is not a pretty job.

But the gear has many teeth. The pawl only has one, so the engine gear should be safe. Guess it's part of a calculated risk.
 
I would disagree. The pawls were made softer for a reason, so they would fail first, and the harder to get to (much less make) engine gear would not. I don't know what the engine gear was made of or how it was treated, but I would find that out, and make the pawl slightly softer.
 
first off find out what type of material BENNELI used back then...it was more than likely grade 5 or grade 8 regular steel...non hardened....but look at their spec sheets or prints....I do this with locomotives like rebuilding a SHAY LOGGING LOCOMOTIVE....its all right there of what they used...how they heat treated if necessary and so on....I would think bennelis are online too

Benelli was/is a minor brand of motorcycle. Getting actual spec sheets from Italy from 50 yrs ago isn't an option. I agree with your opinion that the orig. part wasn't hardened, or at least not properly hardened.
Which is why they keep failing, and why I'm here now.
 
How often do I want to replace the engine gear? Never. Removing/replacing transmission shafts to get at the gear, is not a pretty job.

But the gear has many teeth. The pawl only has one, so the engine gear should be safe. Guess it's part of a calculated risk.

The gear may have many teeth, but if the engine stops at a place where there is a broken tooth in line with the pawl you'll have to put the engine in gear and push the bike a bit to nudge it around to an intact tooth. That can get old real fast and if you take a good stab at the kick starter and engage nothing you can overextend muscles and injure yourself.

I would vote for tough rather than hard so the pawl wears before breaking but still takes some sacrificial wear compared to the gear.

Since you're having to resort to "field expedient" tools starting with one of the tougher alloy 10 x 10 key stocks might make life easier. Perhaps the medium carbon at 64,000 PSI might work well. Just pay careful attention to not leaving tool marks that would weaken the part.

1 x 1mm Key Stock - Fasteners - Grainger Industrial Supply
 
As suggested I would use a material similar to the original. Keep in mind that in this type of situation one material is generally sacrificial to the other. By that I mean you want the easiest to replace and least expensive part to be the one that wears. Replacing the pawl may be a pain, but if it takes substantially more time and costs more to replace the mating gear, the pawl would be the replacement part of choice.

As for hardening you need to know the composition of the mating material. You don't want the pawl harder than the mating gear. If you do it's likely you'll chip the teeth on the gear rather than wear out the pawl. To me case hardening wouldn't be a good choice for this application. It's generally .030" deep which wouldn't last long. Case hardening is generally used to minimize wear on rolling loads not shock loads. In this case the thin case hardened surface could be eliminated by just a few slips of the pawl attempting to engage the gear.
 
So the pawl should be softer (sacrificial) than the gear. Okay.
And case hardening wouldn't be durable. Okay.

Through hardened tool steel it is, but how hard?
Do machine gears/transmission gears have a standard hardness that I can judge by?
 
Benelli was/is a minor brand of motorcycle. Getting actual spec sheets from Italy from 50 yrs ago isn't an option. I agree with your opinion that the orig. part wasn't hardened, or at least not properly hardened.
Which is why they keep failing, and why I'm here now.

back then its more than likely its grade 5....you want I should find the prints for you....they are online...all of them...minor manu or not...everythings online now....and I mean everything....this is a link to the


SERVICE MANUALS
http://www.benelliforum.com/forum/general-benelli-discussion/9924-benelli-manuals.html


heres the ORIGINAL service manuals by CLYMER...buy it IMO
Vintage Two-Stroke Motorcycle (196s-197s) Service Repair Manual
 
I dunno about the case wearing after a short while, not durable? Hmmm..
IME, case hardening is used where you want/need a very hard, wear resistant surface with a soft[ish] core that won't crack/fracture. My Ducati with desmo valve system used case hardened shims to set the valve clearances. Here you obviously don't want wear but you can't have the shim break and fall into the engine. If I made that part I'd probably make it from something like 4140 and then case harden it, maybe with a torch and some Kasenit/Cherry Bomb. Or just use 4140 prehard and leave it 'soft'.
A guess at the hardness of gears: 48-52
JinNJ
 
ScottL - Thanks for the link to the key stock. I was having a tough time finding correctly sized steel (10mm bar) to work with. Thank you again.

JohnnyL - Motorcycle manuals aren't like locomotive manuals. They say nothing of gear composition at all. Zero. Zilich. I've got orig. paper copies of manuals from the era. Cycle mechanics are Not machinists, which is why I'm here. The Benelli forums didn't have the answer other than "the orig. is too soft and fails often."

J. Lewis - Good to hear from a cyclist, but you're voting for case hardening and/or tool steel left in the soft? Now I'm not sure what to do. People are going both ways.
 
Through hardened tool steel it is, but how hard?
For simplicity in hardening and tempering, I'll assume you're using O1 tool steel. If you do an oil quench from cherry red tending toward orange (about 1500F or 815C), the result should be about RC63-65 hardness. If you wanted a cutting tool, you could temper in a kitchen oven, but you want the result much softer than that. Remove the scale from the quenched hardened pawl so you can see the bare metal (sandpaper), and heat the non-working end with a torch, letting the temper colors run toward the working end of the pawl. At 800F/435C, the color will have gone past blue into steel grey, and the resulting hardness should be about RC50. For more accuracy, or for a softer temper, you will need a Tempilstik indicator or similar, because colors on O1 in the required temperature range of 800-1200F will be useless. (RC45 at 1000F/535C, RC40 at 1100F/590C and RC30 at 1200F/650C)

Ideally, you would temper as soon as the quenched part can be held in your hand, and you would hold it at tempering temperature for an hour for every inch of thickness with a minimum of an hour. You won't be able to do that with a torch, but your objective is to make the part much softer than the usual target, so overheating a bit may be helpful rather than harmful. If you use a very low torch and take more time doing the tempering, letting the colors move slowly from the heated end toward the business end of the pawl, that would be much better than tempering with a hot torch.

You might find it useful to start with 1040 or 1045 instead of a full-on tool steel. If you harden 1045 from an orange heat, quench in either oil or water, and temper it to 500F in a kitchen oven the result should be around RC50, with a bronze or brown-yellow temper color. A part the size of your pawl will through-harden in 1045.
 
Now I'm not sure what to do. People are going both ways.
Why not start "soft" and tolerate/expect some wear. Because if you make it hard and you wear the engine gear, you have a major problem. j lewis' suggestion of 4140 prehard seemed to me to be reasonable. Strong and tough. If the gear is in the range of Rc50, then prehard (~Rc30, I think, right?) might be just the ticket. Fully hardened 4140 gets up to Rc 40.

Be interesting to know the hardness of the original part. Good luck.
 
There was a suggestion recently on another forum to temper using an electric heat gun. Though I haven't tried it yet, it may give more control and consistency than a torch. I'm not sure if it would get the part hot enough, but I suspect it would.
 
Be worth digging around the coating world to see if there is something affordable and easily available to improve performance in this sort of situation. If I recall correctly similar material overload problems were one of the reasons why DLC and similar vacuum deposited coatings were developed.

Coating I had done to the innards of my Norton Commander (rotary engined) gearbox seems to work well but I guess I won't know unless I have to pull the box for another reason. Coating I used is often applied to gearboxes on race cars overloaded by highly tuned engines and is said to be the only reason boxes hang together for a season.

Small part like yours should be no trouble to do as it will take up little space when the vacuum chamber is pretty much filled with other folks jobs.

Clive
 
Whoa, whoa. Remember I'm a home, hobbiest. Vacuum chamber special coatings are not an option.
4140 "prehard?" Do you mean annealed, soft as it comes from the mill?

The original part fails too early. Unhardened replacement parts also fail too early. But I don't know if the orig. or the replacement parts were made of tool steel. Let me ask on the Benelli forums, but I am not expecting a deep level of knowledge of this.
 
electric heat gun
I was about to dismiss "electric heat gun" as inadequate, but a bit of reading up on their operating temperatures suggests the high-range on two-heat models might work for this application. You definitely need a gun with a temperature well into the 1000F range, and building a small enclosure for the part out of insulating firebrick or similar would help with heat loss.
Fully hardened 4140 gets up to Rc 40.
Quite a bit harder than that! RC55 is routine for as-quenched 4140; RC59 is not impossible, depending on specific alloy composition.
4140 "prehard?" Do you mean annealed, soft as it comes from the mill?
4140 (or 4142) is available both soft/annealed and pre-heat-treated to just under RC40. The latter is often called prehard, but it's been through a full tempering process, not left hard as-quenched. You can get modest sizes of 4142PHT shipped from onlinemetals.com.
 
I'll be flat out honest: I skipped a lot of this discussion arguing about material and hardnesses, because as a guy who has done a fair amount of repair work on older motorcycles, the only thing this results in is a split transmission shaft.

I'd be willing to put up money saying they split a ton of these shafts during the early years, or testing at the factory, and that making this part sacrificial was their solution to the problem.

Don't stick a grade 8 bolt where a shear-pin is supposed to go: it ends badly :)
 








 
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