Drill Motor Gearbox Reduction Ratios

[archiver]

Posted by Ben Hebert.

Student on team #192, Gunn Robotics Team, from Gunn High School and Sun Microsystems/NASA Ames/Xerox PARC/Nortel Networks/Scitor/Alan Steel/Madco/Vanderbend.

Posted on 2/8/2000 7:04 PM MST



I'm sure this info is out there somewhere, but I was wondering if anyone could tell me the reduction ratio
that the gearbox applies in both high and low gear. Experimentally (ie... screwdriver instead of motor,
counting rotations), we think it is 3-4 times in high gear and 16 in low gear. However, the motor has a
no-load speed of 20000 rpm, and the actual drills these assemblies are associated with claim to have a
no-load, high gear speed of 1200 rpm. This implies about an 8 times reduction, which is very different
than what we experimentally determined. Can anyone tell me what the ratio actually is and explain this
(apparent) contradiction?

[archiver]

Posted by Joe Johnson.   [PICTURE: SAME | NEW | HELP]


Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.

Posted on 2/8/2000 7:35 PM MST


In Reply to: Drill Motor Gearbox Reduction Ratios posted by Ben Hebert on 2/8/2000 7:04 PM MST:



I have not counted the exact number of teeth at each stage in a long while, but I carry around with me the rough numbers for that gear box.

In 'high' the ratio is approximately 20:1

In 'low' the ratio is approximately 64:1

The discrepency is probably due to the fact that your screw driver was not a very good 'sun' for the planets on the first stage. If the planets do not 'roll' around the sun then the ratio you get will not be the proper one.

By the way, Bosch is playing some games with the numbers of teeth on the gears at each stage.

Take the last stage for example:

In college, we were taught that the number of teeth on the ring is equal to the number of teeth on the sun plus twice the number of teeth on the planets.

Counting teeth on the last stage shows 13 on the sun, 14 on the planets, and 43 on the ring!!! Things don't quite add up!

Here is what I THINK they did. They needed to 'enlarge' the small planets and sun in order to avoid undercutting the teeth. Typically, this is done by increasing the center distance between the gears.

BUT... how can you increase the center distance on a planetary set up? increasing the distance between the sun and the planets only moves the ring and the planets closer!

What to do?

Well, suppose they cheat. They remove a tooth from the sun. VIOLA!

Like magic they now have room to enlarge the sun and the planets. They keep the planets gears where they belonged before the stole a tooth from the sun and now there is more room between the centers.

Now, what does that mean to the ring? Well each planet was enlarged about a 1/2 a tooth's worth. That would mean that the ring would then have to be enlarged by a tooth in order to accommodate the planets on opposite sides of the ring.

It wall works out very nicely. Except a poor engineer who actually BELIEVED his professors back at college ;-)

Speaking of college, here is a homework puzzle for all of you:

If the planetary system had been a 'textbook' case, the ratio of a planetary gearbox is (assuming the ring is stationary) W_sun = {(N_ring + N_sun) / N_sun }*W_carrier

What is the ratio in the case where the mfg has stolen a tooth from the sun and given it to the ring?

Joe J.

[archiver]

Posted by Harold Lawrence.

Other on team #458, RoboIndians, from Jefferson High School and NASA AMES.

Posted on 2/13/2000 10:31 AM MST


In Reply to: Drill Motor Gearbox Reduction Ratios posted by Ben Hebert on 2/8/2000 7:04 PM MST:



I measured the no load RPM in both high and low with a direct reading mechanical
Tach. In low gear it was 250. High was 750. These are actual values tested on
both motors +,- 2%.

Harold

: I'm sure this info is out there somewhere, but I was wondering if anyone could tell me the reduction ratio
: that the gearbox applies in both high and low gear. Experimentally (ie... screwdriver instead of motor,
: counting rotations), we think it is 3-4 times in high gear and 16 in low gear. However, the motor has a
: no-load speed of 20000 rpm, and the actual drills these assemblies are associated with claim to have a
: no-load, high gear speed of 1200 rpm. This implies about an 8 times reduction, which is very different
: than what we experimentally determined. Can anyone tell me what the ratio actually is and explain this
: (apparent) contradiction?

[archiver]

Posted by Harold Lawrence.

Other on team #458, RoboIndians, from Jefferson High School and NASA AMES.

Posted on 2/13/2000 10:31 AM MST


In Reply to: Drill Motor Gearbox Reduction Ratios posted by Ben Hebert on 2/8/2000 7:04 PM MST:



I measured the no load RPM in both high and low with a direct reading mechanical
Tach. In low gear it was 250. High was 750. These are actual values tested on
both motors +,- 2%.

Harold

: I'm sure this info is out there somewhere, but I was wondering if anyone could tell me the reduction ratio
: that the gearbox applies in both high and low gear. Experimentally (ie... screwdriver instead of motor,
: counting rotations), we think it is 3-4 times in high gear and 16 in low gear. However, the motor has a
: no-load speed of 20000 rpm, and the actual drills these assemblies are associated with claim to have a
: no-load, high gear speed of 1200 rpm. This implies about an 8 times reduction, which is very different
: than what we experimentally determined. Can anyone tell me what the ratio actually is and explain this
: (apparent) contradiction?