I am doing a little off season project and experimenting with different ways to mount the drive motors to save space. My current design involves a set of spiral bevel gears, with one mounted directly onto the CIM shaft and the other on an axle supported on both ends. I don’t have any experience using or mounting bevel gears, but during my research one source said that these type of gears puts a strong horizontal force on the shafts. All I am wondering is weather or not the CIM will be able to handle this force, or if I need to support the other end of its drive shaft. I don’t have many specifics yet, but the gears will most likely be a 3:1 reduction.
PS. these plates will be CNC’d so everything will be precisely aligned, and built strong.
Any suggestions or thoughts will be greatly appreciated.
I suggest adding a thrust bearing of some sort, as the CIM does not appear to be designed to take a lot of thrust load. Also it will help keep the bevel gears properly aligned.
Do you have any suggestions on where or how to mount the thrust bearing. There is little room at the end of the CIM shaft because of the proximity of the other bevel gear, so could I mount the thrust bearing inbetween the CIM and the bevel gear, or would that be redundant. Or else is there some sort of bearing device that could attach the end of the CIM shaft to the axle the bevel gears are driving, which would support the CIM shaft? Any ideas?
Do you have a picture or drawing of the gear you plan to use?
A thrust washer (bronze, plastic, fiber) between the gear and the CIM (or the plate that you bolt the CIM to) could do the job, assuming the gear has a flat rear face.
You could also take apart a motor and see what the thrust washer/bearing looks like.
Jim’s already addressing the thrust issues with you, so I won’t be addressing that here. My concern is the gear ratio. 3:1 is a little low unless you plan on using very small wheels. (I’m assuming direct drive from the second bevel gear).
Here is a good general guide line when working with the CIMs to make sure you don’t overload them.
The KOP Toughbox is a 12.75:1 ration gearbox. In addition, the output and wheel sprockets give an additional 22:15 reduction. So, 12.75*(22/15)= 18.7:1 total reduction. Now, the KOP wheels are 6in. dia. Therefore, 3.1417*6= 18.8 in circumference.
One quick way to look at this is, design for approximately one revolution of the CIM for 1 inch travel. This is for a 150 LB. robot and it is only a rule of thumb.
Now, from experience, +/- 25% on these numbers works, but is not always optimal. Make sure you take into account what you goal is: Speed, Torque, Longevity etc.
Your milage may vary.
Thank you Bill, but the wheels are actually being powered via timing belt, so the pulleys I will be using have a further reduction of four to one, giving me a final reduction of 12:1 which is what our team has used before with much success. I am confident in my overall design, but am just looking for help on how to mount the thrust bearings or washers, due to the limited space, and also i am trying to save as much weight as possible.
I have attached a picture of bevel gears which are similar to the ones i will be using. In the current configuration the CIM will be bolted to a 1/4" plate (which is overkill, but is necessary to mount it at the 90 degrees required to save space. This mounting plate comes almost flush to the shoulder around the output shaft on the CIM. I am not sure how thrust washers work or are mounted, so please explain further.
Using a thrust washer requires smooth, sort of hard surfaces to bear against. Another approach is to use a thrust bearing like 6655K14 (mcmaster carr).
Ohh, you are saying to use these bearings or washers to absorb the force applied on the bevel gear that is inline with the shaft (from one end of the CIM to the other) as opposed to a side to side motion. Ok so I can use a thrust bearing between the gear and the face of the CIM, but do I need to support the end of the CIM shaft somehow to prevent any side to side displacement?
this site has some good info on the spiral bevel gears. If you scroll down to the section on them, it has a diagram which describes (and even shows how to calculate) the loads that would be created.
The force they mark as Fp will be what will probably require a thrust bearing to compensate for. This would be placed between the face of the motor (or the plate that the motor mounts to) and the pinion itself. It’s job is to take that load and apply it to the face, so that the shaft isn’t taking that load.
You’ll see the Force Fs also applies a small amount of thrust loading, but more so, pushes normal to the shaft. Depending on how large this is, you may need to support the end of the CIM shaft with a normal bearing. Supporting it at the face of the CIM would do a little bit, but not very much. I would assume you need to support it on end.
The bearings in the CIM motors are not designed for any force to be applied as a side load and there is no real thrust bearing to prevent the shaft of the motor from pushing into the motor. The CIM bearings are simple bronze bearings. It is best to have an external bearing point on the motor shaft for side loads. Without an external bearing point, the CIM bearing will exhibit significant wear and the gears will pull out of alignment.
So that means yes, best practice would have the end of the CIM shaft supported by a bearing, as well as a thrust bearing somewhere to counter the tendency of the gear to push away from it’s mate.
Do some research on an automobile rear differential, it uses a setup similar to what you are doing.
Sort of…but car ring and pinion gears need to last a couple hundred thousand miles/10 years or more, and transmit a lot of power. They use a pair of tapered roller bearings to support the pinion, and the pinion gear and shaft are one piece. The bearings are installed with preload rather than having end play.
Ok, thanks alot, what I think I will do is use a thrust bearing between the bevel gear and the plate the motor is mounted to, and then try to squeeze a ball bearing in on the end of the CIM shaft. However due to the limited space at the end of the CIM shaft, I might have to use either a thin section bearing or a bushing, and mount either the bearing or bushing to a plate parallel to the motor mounting plate, and have them separated by spacers. (I was inspired by this picture) Does anyone have a good source for thin section bearings that isnt too expensive? And if not a bearing, do you think the busing will be able to handle the high rpm’s without causing much friction? I am hoping to receive the catalog from Toronto Gear soon, so I can continue the CADing, and once i have some finalized drawings I’ll post them. Thanks alot for your help.
On second thought, would a tapered roller bearing situated between the bevel gear and the motor plate do the role of both? Aka, the bearing would be press fitted into an additional plate attached to the front of the CIM, or is a support at the end of the shaft still needed?
