Designing a sprag clutch assembly


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    Default Designing a sprag clutch assembly

    This is for a large rc helicopter. The sprag is used on the main drive gear to drive the rotor head and allows the rotor to free wheel for autorotations when the motor is shut down. A common sprag size for this size model is 15mm ID, 23mm OD, 12mm tall. Typically the mainshaft is 12mm and a steel sleeve is used as the inner race of the sprag. The outer race of the sprag is also typically a sleeve, 23mm id, 27mm od, 12mm tall, and gets pressed or loctited into an aluminum hub that is bolted to main drive gear.

    The complication I'm running into is deciding what to do about this inner and outer sleeve. I'm trying to source something premade that will work for these sleeves, but having a hard time.

    This is the sprag I will be using:

    http://www.rcbearings.com/products/fe422z-one-way-bearing.html

    I found specs on another site that specify the requirements for the inner and outer race. Needs to have a surface finish of 2.5um and hardness of hrc60.

    Hardened 52100 seems like a common choice for this type of thing, but don't think it's something I can do myself. I think I could hit the dimensional and surface requirements in annealed 52100, but I've read that the hardening process is specifically tricky with this material and requires a decent amount of material left for grinding after hardening. I've thought about building a tool post grinder setup since I need it for other things, but grinding of hard 52100 is also not so easy from what I've heard.

    For the first prototype batch, I'll probably only need 5 sets of inner and outer race, not sure what the cost would be to have these custom made. I'm kind of new to this whole process. Up until now I've either made my own parts, or found existing ones to work. Wouldn't even know who to contact to make these type of parts.

    Anyway, looking for advice on how to approach this. Maybe there are pre existing races that I just haven't found. Otherwise not sure if I should try doing part of this myself, maybe turn the annealed parts and send out for both hardening and grinding, or just outsource the whole thing.

    Any advice is appreciated.

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    Community Moderator Jim Dawson's Avatar
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    Default Re: Designing a sprag clutch assembly

    Take a look at the VXB offering for sprag bearings https://www.vxb.com/One-Way-Bearings-s/231.htm The outer sleeve is integrated, but most require an inner sleeve. There is one that has both and looks like a normal ball bearing. I'm guessing the major bearing manufacturers have offerings that would work. Maybe check with your local bearing vendor.

    I have seen sleeves that might work for the shaft. Grinding your own would be pretty easy if you had a cylindrical grinder but without that it might be a bit tricky. Maybe could be done on your machine, but would require an interesting setup.

    Jim Dawson
    Sandy, Oregon, USA


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    Quote Originally Posted by Jim Dawson View Post
    Take a look at the VXB offering for sprag bearings https://www.vxb.com/One-Way-Bearings-s/231.htm The outer sleeve is integrated, but most require an inner sleeve. There is one that has both and looks like a normal ball bearing. I'm guessing the major bearing manufacturers have offerings that would work. Maybe check with your local bearing vendor.

    I have seen sleeves that might work for the shaft. Grinding your own would be pretty easy if you had a cylindrical grinder but without that it might be a bit tricky. Maybe could be done on your machine, but would require an interesting setup.
    The problem with those needle bearing clutches is they don't have the same torque capability nor are they as reliable as the sprag. Some helicopter manufacturers use them, but generally in smaller models. In 700 size models, which is what I'm building, the power system can peak at around 12kw (about 16hp) or sometimes higher. The brushless motors in these are insane. 12lb helicopter can accelerate 0-100mph in literally the blink of an eye.

    So if I build a cylindrical grinder to grind the hardened parts myself, what about the hardening of the 52100? I basically know nothing about the hardening process, is that something I can do? Or relatively cheap to have someone else do it?

    Also, I'm kind of just assuming I'll be able to turn the annealed 52100, but not positive. So far the hardest thing ive turned on my machine is 303, but the surface finish and tolerance was excellent. Not sure how the 52100 would compare to that.

    Also, are there any other options for producing these sleeves with the required surface finish, tolerance, and hardness that could be easier than turning, hardening, grinding? Maybe some type of plating to get the required surface hardness? Not sure if I mentioned the required tolerance, but it's about 4 tenths.



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    Community Moderator Jim Dawson's Avatar
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    Default Re: Designing a sprag clutch assembly

    I think my local heat treaters have a $50 minimum charge. It is possible to do it in the home shop, but requires the proper equipment and a learning curve. A controlled atmosphere or vacuum furnace is the best option. Annealed 52100 should turn OK, I wouldn't expect any problems.

    Grinding is really the only option, even if you had them hard chromed. A cylindrical grinder, even a small one, is a large machine, normally weighing in at several thousand lbs. With careful setup it might be possible to do what you want with a tool post grinder in a lathe.

    Jim Dawson
    Sandy, Oregon, USA


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    Quote Originally Posted by Jim Dawson View Post
    I think my local heat treaters have a $50 minimum charge. It is possible to do it in the home shop, but requires the proper equipment and a learning curve. A controlled atmosphere or vacuum furnace is the best option. Annealed 52100 should turn OK, I wouldn't expect any problems.

    Grinding is really the only option, even if you had them hard chromed. A cylindrical grinder, even a small one, is a large machine, normally weighing in at several thousand lbs. With careful setup it might be possible to do what you want with a tool post grinder in a lathe.
    Ok, 50 bucks isn't bad. I think I'll start by ordering a chunk of 52100 just to see if I can turn it. If it goes well, I'll go from there.

    As for the grinding, here's my thought, one thing I definitely need to add to my mill turn setup is a small horizontal milling spindle on the gang tooling on same axis as turning spindle so i can do light duty milling on the face of certain parts. For this i was thinking about grabbing a small 500 watt er11 spindle will hold tools up to 3/8 shank. If 500 watts is enough power to do some light grinding, it should also work great for that. I would think the spindle will have decent bearings in it, balanced, low runout. Not sure what kind of rpm I would need for this, but it will go 10k rpm max. For the id, I'll need pretty small grinding wheels anyway. For od, I'm sure it's more common to use bigger grinding wheels, but I'm sure the available torque of the 500 watt spindle isn't great so I'll probably need a relatively small wheel for od grinding as well. But anyway, I need this small horizontal spindle whether I use it for grinding or not, so might as well try it for that.



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    Default Re: Designing a sprag clutch assembly

    500W should be enough for a small wheel. 10K should be fine. Make sure to buy wheels that are rated for at least that speed. Grinding is normally done with a wheel surface speed of around 5000 FPM or so.

    Jim Dawson
    Sandy, Oregon, USA


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Designing a sprag clutch assembly

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Designing a sprag clutch assembly