Shafts and bearings

Building a small gear train with a good efficiency is not so easy. One need adequate components and tools.We give here several hints and DIDEL provides the components. See our price-list if you are not coming from there.

Loose gears

Intermediate gears may be loose on their axis. Play is 0,02mm minimum and with POM gears, no lubrification is required. If the gear has a 0.78 mm hole, a steel axis of 0.75-0.76mm or a rivet of 0.76 mm in diameter is adequate. Rivets are an easy solution in many cases, because they maintain the gear in place. The carter must be precisely drilled (0.72 to 0.75 in soft plastic) so the rivet will fit hard, but not too hard in its hole. It is recommended to taper the rivet to facilitate its insertion, and avoid that the sharp rivet edge will enlarge the hole, specially with plastics.


Axis with a carter

Axes are good and easy if the gear is maintained at both ends by an adequate carter. The carter holes are only slightly smaller than the axis, so the axis can be removed easily. If any play, a drop of glue can be used to maintain the axis in place.
If the gear overhangs, one can stop the gear with a Quicklock washer (0.8 mm axis) or, if there is no axial force, a piece of plastic tube taken from the appropriate electric wire.

Elastic circle
For axis of 1.5 mm and larger, on can use elastic circles for smooth axis, that need a special plyer for insertion. If the axis has to carry a wheel, a simple solution is to press-fit the wheel on the last gear. If the wheel has been molded over a gear, the torque can be enormous.
Tight gears

Tight gears on a small diameter axis cannot transmit an important torque. POM gears cannot be glued, and the pression of the gear to the axis is low compared to metal to metal. Inceasing the difference in diameter to increase the pression may just result in a deformation of the gear.
For getting a better torque, one should drill a large hole in the gear, insert a brass sleeve (bore in gear 0.02 to 0.03 mm smaller than the brass tube), that is press-fitted through the axis (bore in brass 0.02mm smaller).

Brass bearings
Brass bearings are cheap and easy to implement. They are press-fitted in the carter, with a 0.02 to 0.05 mm compression (the bearing will not deform even with high insertion force). Their precise inside diameter match with steel axis with a minimum play. Watchmaker oil is recommended: the form of the bearing is designed to keep the drop of oil in place.
Ball bearings
Miniature ball bearings are expensive. They have to be used when the radial effort is important. For ultra-light planes, they have the advantage to permit a carbon rod axis.

You like miniature ball bearings? André Guignard built at the EPFL in 1998 a nice demonstrator for RMB. The butterfly wing mechanism uses eight 3mm bearings, plus the two ones inside the 3 mm smoovy motor. If you want to see how it works, there is a beautifull 7 Meg video in MPG format.

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Last update: 08.05.02/jcz