I recently received my RC motor and it appears the motor's shaft is round and doesn't have any "key holes". How would one secure a timing-pulley/sprocket to such a creature to withstand the 200+ lbs. of force I think it'll be experiencing? My timing pulley comes with a "grub screw"/"set screw", so would just tightening it to the motor's round shaft be sufficient? It doesn't seem like it would...
How to secure a sprocket/timing pulley to a shaft?
- Thread starter swbluto
- Start date
boostjuice
10 kW
swbluto said:
The output shaft of the RC motor wont have 200+lbs of torque. You may have 200+lbs of torque at the final speed reduction transaxle, however the torque at the motor shaft is small, but power/rpm is high. The smaller torque at the motor is multiplied by the speed reduction at each transaxle. That means for each reduction stage transaxle, the torque loading on the pulley/sprocket will be larger than the last and hence should be secured appropriately to take the force. However the precise balance of each sprocket/pulley becomes less of a concern for each stage as rpm drops.
That doesnt mean you dont want a secure fit to the shaft.
Id definately be filing off a flat of at least 20mm for a line of grub screws to torque down on the motor shaft assuming your pulley has a round bore of matching diameter. That way you can get secure grub screw purchase at several points. However at the speed those things spin balance may be an issue, so id be removing just enough material for a flat that matches the diameter base of the grub screws. The removed material weight should be attempted to be a close match to the extra wieght of the grub screws tightening onto it. That way you minimise the chance of vibrations occuring due to balance problems.
It is also possible to drill shallow holes into the shaft slightly bigger than the base of the grub screw, this way the pulley will be more secure to both torsional and thrust loads.
To file or drill the shaft, secure the shaft itself in a bench vise. That way the shaft wont spin and the motor bearings wont take the cutting load of the tool being used. Cutting fluid/WD40 would be recommended if drilling as motor shafts are often hardened steel and will quickly overheat/blunten unlubed drillbits.
Malcolm
10 kW
Sounds like you might want a keyless bushing. They can cope with a surprising amount of torque.
http://shop.fennerdrives.com/cgi-bin/lansaweb?webapp=WHOMPGE1+webrtn=LWBOE01D1+ml=LANSA:XHTML+lang=ENG
These limit the smallest pulley you can use, since you have to allow for the bushing as well.
You can buy them online under the "shop" link.
http://shop.fennerdrives.com/cgi-bin/lansaweb?webapp=WHOMPGE1+webrtn=LWBOE01D1+ml=LANSA:XHTML+lang=ENG
These limit the smallest pulley you can use, since you have to allow for the bushing as well.
You can buy them online under the "shop" link.
I used a Dremel grinder to make divots in the shaft on my BMC motor and the sprocket has two set screws. It works best if you have two set screws at 90 degrees.
To locate the divots, I tightened the set screws securely, then removed them and looked for the dents in the shaft they left. I then ground the area around the dents. Use a little loctite on the set screws and between the sprocket and the shaft.
I've been running this setup at up to 5kw with no problems. The shaft is 12mm.
To locate the divots, I tightened the set screws securely, then removed them and looked for the dents in the shaft they left. I then ground the area around the dents. Use a little loctite on the set screws and between the sprocket and the shaft.
I've been running this setup at up to 5kw with no problems. The shaft is 12mm.
You can use loctite 668 as long as you're bonding something that has a tolerance within .25mm. It's actually meant for what you're doing. The negative of it is that it's semi permanent. Once it is set it's a bear to get it off. You either have to heat the part above 450F or I've also heard, but not tried using a shot of liquid nitrogen which is suppose to cause it to crack and be able to be pulled off.
Heh. Heh. It doesn't seem like drilling a ~6mm hole(The timing pulley has a 5mm set screw) would work too well on a 10mm shaft and my drill press doesn't seem to work on it, as the drill bit seems to be deflecting from the motor shaft's roundness whenever contact is made. Would securing it by creating a "flat area" for the single grub screw be enough? The timing pulley has only one grub screw hole.
Other than that, I'm thinking about creating a thinner hole through the timing pulley and the motor shaft and just use a smaller pin(Like say, a thin tension pin.), assuming the shaft is drill-able at all with my equipment. I want to make it "sort of" temporary as I want to convert from my belt to a chain system to try it out, and I want to be able to convert back from the chain to belt in case it seems to be worse.
Other than that, I'm thinking about creating a thinner hole through the timing pulley and the motor shaft and just use a smaller pin(Like say, a thin tension pin.), assuming the shaft is drill-able at all with my equipment. I want to make it "sort of" temporary as I want to convert from my belt to a chain system to try it out, and I want to be able to convert back from the chain to belt in case it seems to be worse.
Hey, Just thought I would jump in for a sec. If you still want to drill the shaft create a flat spot as long as the diameter of the hole you want to drill. Center punch on the flat and the drill won't wander. It would work even for the pin idea and you wouldn't have such a big hole. Good luck!
You have to keep in mind that when designing gear reductions, the torque is inversely proportional to the speed. As power (watts) = volts x amps in electrical design, the same power is transmitted at 72V 30A as 36V 60A. Similarly for mechanical systems, power (watts) = torque (Nm) x velocity (rad/s). The same mechanical power is transmitted in a shaft at 1000 RPM, 10 Nm and 100 RPM, 100Nm.
So the actual torque at your motor is not going to be all that much, since it's spinning at several thousand RPM. For a quick and dirty calculation, say your motor is 85% efficient and it's drawing 1200W of electrical power. So .85 * 1200W = 1020 W of mechanical power. If the motor is spinning at 4000 RPM, to use SI units you must first convert RPM to radians/sec:
4000 revolution/min = 4000 * (2pi) = 8000pi radian/min
8000pi radian/min = 8000pi / 60 = 133.33pi radian/sec or 418.8 rad/s
Now Power = torque * velocity, so torque = power / velocity. 1020 W / 418.8 rad/s = 2.43 Nm. That's it. No problem for set screws.
Now if you have, say, a 16:1 reduction, then your final output is going to be 418.8 / 16 = 26.1 rad/s, and
2.43 Nm * 16 = 38.9 Nm. Kind of a problem for a single set screw.
Just to check, final output power = 38.9 Nm * 26.1 rad/s = 1014 W. I chopped off some decimal points, but it's about the same, so we're good.
I hope this helps some. Let me know if anything is unclear and I'll try to explain it better.
So the actual torque at your motor is not going to be all that much, since it's spinning at several thousand RPM. For a quick and dirty calculation, say your motor is 85% efficient and it's drawing 1200W of electrical power. So .85 * 1200W = 1020 W of mechanical power. If the motor is spinning at 4000 RPM, to use SI units you must first convert RPM to radians/sec:
4000 revolution/min = 4000 * (2pi) = 8000pi radian/min
8000pi radian/min = 8000pi / 60 = 133.33pi radian/sec or 418.8 rad/s
Now Power = torque * velocity, so torque = power / velocity. 1020 W / 418.8 rad/s = 2.43 Nm. That's it. No problem for set screws.
Now if you have, say, a 16:1 reduction, then your final output is going to be 418.8 / 16 = 26.1 rad/s, and
2.43 Nm * 16 = 38.9 Nm. Kind of a problem for a single set screw.
Just to check, final output power = 38.9 Nm * 26.1 rad/s = 1014 W. I chopped off some decimal points, but it's about the same, so we're good.
I hope this helps some. Let me know if anything is unclear and I'll try to explain it better.
The set screw of concern is the one attached to the motor's shaft. Since it's known that the motor has an initial torque of 5 N.M., then the screw being positioned 5mm=.005 meters away from the axis of rotation will have a force of 5Nm/.005m = 1000 Newtons ~ 224 lbs of force. This "initial force" becomes more of a concern as you travel under harsher circumstances(Heavy rider, going up a hill, etc.) which is what I'm designing this scooter for.
Yes, that's a good point I left out. The smaller your radius, the higher the actual force experienced at the point of contact. Which is why bigger shafts are able to transmit higher torques...
Torque is just a convenient way to work with rotational mechanics but it's important to understand what's going on behind the numbers!
I like buzzfirst's idea too. If you try it out let us know how well it works.
Torque is just a convenient way to work with rotational mechanics but it's important to understand what's going on behind the numbers!
I like buzzfirst's idea too. If you try it out let us know how well it works.
The sprocket on an original Zappy uses a roll pin that goes through a hole in the axle. It is also made for an 8mm shaft. Cross drilling a hardened motor shaft may be extremely difficult. If you can get a drill that takes it, keeping the hole perfectly aligned is also difficult.
If you had something like the Zappy sprocket, it could be used as a drill guide.
The roll pin worked OK, but was difficult to remove.
If you had something like the Zappy sprocket, it could be used as a drill guide.
The roll pin worked OK, but was difficult to remove.
I agree a hardened shaft would be tough. One of the additional advantages of creating a flat on the shaft comes in to play when the shaft you are drilling is case hardened, meaning the outer shell of the shaft is hardened but the center is still just steel. Once you grind/file through the thin hardened layer a normal drill will work well until you get to the other side and then it will probably go on through and the drill will be dulled a bit. If you really really want the hole in a completely hardened shaft solid carbide drills are available, they aren't cheap and will break at the drop of a hat if you get them out of alignment. They cut hardened steel like butter however if you try one in non hardened steel they will just break. Good Luck!!
Thanks for everything! I decided to avoid the tension pin and just drilled a small hole. It worked beautifully filing it flat and creating a small indentation for the drill's point.
Anyways, I now have the scooter semi-complete and I have a build thread now.
Anyways, I now have the scooter semi-complete and I have a build thread now.
