General geartrain questions

[uh60rascal]

I am putting together a spur gear train to shift the axis of rotation several inches below the output shaft of a gearbox. The top gear is the pinion (20 teeth) then a 48-tooth idler, which is mounted on two flanged ball bearings to allow it to rotate freely on a fixed shaft, and finally another 20-tooth gear attached to my drive shaft. They are all Module 1. (Image attached.)

Having never designed or built a gearbox before, I have several questions that I have been unable to find answers to (or else I've been looking in all the wrong places):

1. I do not expect these gears to have any axial load placed on them, but will they if they are not lined up perfectly? Do I need to have some way of retaining the gears on the shafts axially? For the idler, I have a retaining ring on one end of the shaft, but so far that is all. That ring is in contact with the flange bearing rather than contacting the gear itself. I am hesitant to use washers, spacers, e-rings etc due to the friction they would introduce into the system... or am I overestimating their influence?

2. How much of a gap should I leave between the gear ODs and the wall of the housing? Max speed will be around 2200 RPM so I want to make sure the grease does not go flying off. Another gearbox I saw had an extremely small gap between the wall and gear surfaces. I was thinking 1 or 2 mm gap?

3. I don't currently have any play in my design to adjust the distance between the shafts. I know what the mounting distance should be center-to-center, so should I be OK if I am accurate with my fabrication or is this a poor design quality?

I know there are a lot of questions, but even if you can only answer one part it would help me greatly. Thanks!!

~Ashley

 

[BrianE22]

1) Yes, you will need to restrain them axially. Misalignment and deflections can produce (usually very) small axial loads. Needle Thrust Bearings might be overkill but they would keep the friction low. E-rings or snap rings would contact the gear only near the hub (small radius for troque) so that frictional torque wouldn't be much.

3) If the amount of backlash isn't important you shouldn't need anything special other than to tell the gear cutter the center distance and about how much backlash you need.

 

[tbuelna]

uh60rascal,

You have low pitch line velocities, so a 2mm gap between the gear OD's and housing is OK. It is probably even beneficial with the bottom pinion, since the grease will tend to collect there and then be picked-up by the pinion and transferred around the mesh.

The type of bearing used on the idler is probably not important. In fact, plain bearings would even work well. Spur gears don't produce any appreciable axial forces, and your idler with diametrically opposed pinions will produce very little radial load. So the friction losses in plain bearings would likely be acceptable.

Good luck.
Terry

 

[uh60rascal]

Thank you, Brian and Terry. I appreciate your help.

The top gear shown in the image is attached to the output shaft of my gearbox. Is there an alternative to trying to machine a groove in that shaft and insert e-rings? It seems like the key would get in the way, and also I don't want to risk damaging the gearbox in the process.

 

[uh60rascal]

So is there a way to use retaining rings and keys together? Or an alternative approach? This seems like an extremely elementary question, but I can't seem to find anything about it on Google, Marks', or Machinery's Handbook.

 

[tbuelna]

uh60rascal,

Is there an end cover that is not shown? Or are your gearshafts really cantilevered?

Terry

 

[uh60rascal]

The bottom shaft (blue) is supported by bearings that are not displayed, but the other two gears are cantilevered. I don't know if I effectively communicated the scale of this design, but the gears have only a 10mm face width with a 5mm hub (except for the idler, which is hubless). So they aren't really hanging out there too far...at least I don't think so.

Thanks!