Gear Bore sizing? 1

[mrpi]

I'm using three straight spur gears to move a gearbox away from a pivot. This is in a low speed lift application, and the gearbox OD is larger than the allowable pivot diameter. It basically just flips a [heavy] panel 180°.

Backlash is not critical as it moves from stop-to-stop, but I am a little concerned that it doesn't 'flop' around too much at the tip-over point when the load reverses.

Currently all the gears are the same, so no ratio thru the transmission.

Anyways, the idler gear I need to offset slightly to gain clearance at the corner so its going to see some radial force. (see image)

Is there a general rule for the maximum bore size thru a gear? X-pitch heights away from the root?

Also, is there a preferred bearing style to use in the idler bore? Two individual radial ball bearings? In the past I had designed an idler gear into a system using a single double-row bearing that was a light shrink-fit into the gear bore. It has been working very well, but this system see much higher loads and very low speeds (3rpm). Or is the double-row bearing well suited to this application?



Beat to fit, paint to match.

 

[tbuelna]

mrpi,

First question- is it absolutely necessary to have the bearings located directly under the gear rim? If you have the axial space I'd suggest moving the bearings apart, to outside of the gear face. This will allow adequate material backing up the gear rim.

As for the max bore diameter thru a gear, a good rule of thumb for the absolute minimum rim thickness is 1.0 tooth whole depth below the root diameter. But in the case of your idler gear with somewhat diametrically opposed contacts, you would also need to carefully evaluate the additional stress effects resulting from the lack of radial stiffness in a gear shaped like a thin-wall cylinder. The cylinder wall is constrained from deflecting radially inward by the bearing, but there is only the relatively low stiffness of the cylinder wall to prevent any radial deflection outward. Plus there is also the asymmetric loads around the rim. The drive side sector of the rim is heavily loaded in compression, while the coast side sector is not.

A final suggestion- since your rotational speeds are modest and you have no real axial loads, and if efficiency is not a big concern, you might want to consider plain bearings. A plain bearing would provide the best combination of radial load capacity and minimum radial space requirement.

Good luck with your project.
Terry

 

[mrpi]

Thanks for the reply Terry,

It is not necessary to have the bearing located 'within' the gear, although I am trying to minimize axial stack height.

I guess I did not include any dimensions, but these are 5in Pitch Diameter gears so I can fit a big bearing in there and still preserve wall thickness.

I had not considered plain bearings!



Beat to fit, paint to match.