pic: 571's new gearz loose weight

[JessR]

For those interested, I posted a pic in the gallery [2004 > robots] showing some detail of the shifter and driveline.
Here:
571 Tranny - shifter details

[sanddrag]

How are the white plastic gears affixed to the shaft on which they lie? Thanks.

[JessR]

Originally Posted by sanddrag

How are the white plastic gears affixed to the shaft on which they lie? Thanks.

Planned: Key and setscrew for each of the 4 plastic gears.

We expected gears as plastic with a metal bushing in the hub large enough to bore & modify. Well, the gears we got in time for ship date were different from plan, so we had to improvise a bit.

Actual:
- The Mod .7 metric gear (meshes with Bosch drill motor) is all plastic, so it has just a key with no set screw. This worked out fine, with the gear hub covered at both faces, the key is captured & can't come out the end of the hub.
- The other 3 plastic gears, I think 32 DP, (meshes with Fischer Price motor) are mixed - have either key and setscrew, or just setscrew.

[JessR]

As a follow-up thought, these improvisations were an interesting experiment, though one we didn't want!
As a brand new design, we were worried about a number of things - plastic teeth stripping, setscrews, tooth binding from sideplate movement, shifter engagement etc. etc.
In practice driving, the only problem was a setcrew-only gear loosened up.
We tightened all setscrews and check periodically at competitions - everything has stayed tight since then through the finals in two regionals.
Metal parts - All the metal gears and output sprockets (either keyed-only, keys captured, or key-and-setcrew), have locktited setscrews. Bearings use a slip fit in shafts & Loctite retaining compound, made assembly very easy.
[remember - For Metal parts - Locktite is your friend. DON'T use Locktite 242 on plastic, it will crack/melt many plastics.]

So we are fantastically happy with the result, as these transmissions were designed from scratch in January, and completed, modified and running a week before ship date. A bit risky for something this complicated but it came out well.
We are looking to simplify and improve the design for next season.

PS,
We have prototype drawings for the parts on AutoCAD - if you want them, email me.
I'm excited to see of course how we do at Championships tomorrow (Archimedes)

Later, Jesse.

[sanddrag]

Originally Posted by JessR

Actual:
- The Mod .7 metric gear (meshes with Bosch drill motor) is all plastic, so it has just a key with no set screw. This worked out fine, with the gear hub covered at both faces, the key is captured & can't come out the end of the hub.

What size bore, keyway, and length through bore? If I could just get those three things that would be great. Thanks.

[JessR]

Originally Posted by sanddrag

What size bore, keyway, and length through bore? If I could just get those three things that would be great. Thanks.

From our drawings;
MODULE .7, 48 TOOTH ACETAL GEAR
(SDP PART# A 1M 2MYZ07048)
The gear was bored for a 5/16 dia shaft with 3/32 key
(the standard key size for 5/16 shaft)
& hub was faced off to .278 length thru bore to fit our space requirements on the shaft.

SDP is Stock Drive Products.

PS How is this useful to you?
Jesse

[sanddrag]

Originally Posted by JessR

From our drawings;
MODULE .7, 48 TOOTH ACETAL GEAR
(SDP PART# A 1M 2MYZ07048)
The gear was bored for a 5/16 dia shaft with 3/32 key
(the standard key size for 5/16 shaft)
& hub was faced off to .278 length thru bore to fit our space requirements on the shaft.

SDP is Stock Drive Products.

PS How is this useful to you?
Jesse

Well, I knew the gear comes from sdp-si already but I find the other information extremely useful. Thanks so much. Do you think something like a 8mm bore with 2mm keyway and .24 length through bore would hold out? The actual keyway (length and width) would only be slightly smaller than what you are using. I would be gearing the Drill down to match the Chia.

Also, thanks for the offer of AutoCAD drawings but that I don't really need them. However, I would like to see a picture of how the Fisher price transfers through the plastic gears to the metal ones. Did you say there were 3 plastic gears for the FP geartrain? I couldn't see detail on that in any of the previosly posted pics. Thanks a bunch. It is a truly wonderful design.

[Salik Syed]

What is the gear inbetween the 2 small gears that mesh with the bigger hi/low gears for?

[JessR]

Originally Posted by Salik Syed

What is the gear inbetween the 2 small gears that mesh with the bigger hi/low gears for?

Let me define the shafts, then you'll see.

There are 4 shafts in this tranny:

1. idler shaft
We also call it the synchronizer shaft, it transfers power from the drill motor to the input shaft. Using this extra shaft allows us to locate the drill motor right next to the fp motor and facing in the same direction.
The drill motor drives the idler shaft at about a 3.5:1 reduction. The idler shaft is geared 1:1 via a pair of plastic gears onto the the input shaft
2. Input shaft
(It's next to the fischer price motor in the foto) - This combines both drill & fp motor inputs onto a common shaft. THe fp motor drives a plastic gear on the input shaft. A second plastic gear is driven 1:1 by the drill motor idler shaft as described above.
3. Intermediate shaft.
About a 3:1 reduction from the input shaft, and drives onto either high or low gear on the output shaft.
4. Output shaft.
That's the one with the sprocket on it and the shift fork/hex sleeve between hi & low gears.

So - the gear you're asking about is on the intermediate shaft, and receives power from the input shaft at about a 3:1 reduction.

A side note, our #1 design goal here was to have the motors completely inside the tranny and have the motors run the same direction on both sides of the robot (the l & r side trannys are identical and face the same direction on both sides of the robot - we just use the other side of the output shaft).

Well - hot news from Atlanta -
I just spoke to the team in Atlanta, we're doing well but had a motor problem a couple of hours ago
- Someone's appendage came inside our chassis and smashed a drill motor brush housing, ouch!!
They finished the round on just on the fisher price, but the poor thing smoked at the end - just finished an emergency motor transplant with 4 new motors in time for the next round......(the tranny's are held in by about 6 screws each, so they come out pretty easily, thank goodness)

Hope you're teams are having fun if in GA.
Jesse.

[JessR]

Originally Posted by sanddrag

Well, I knew the gear comes from sdp-si already but I find the other information extremely useful. Thanks so much. Do you think something like a 8mm bore with 2mm keyway and .24 length through bore would hold out? The actual keyway (length and width) would only be slightly smaller than what you are using. I would be gearing the Drill down to match the Chia. ...

Sandrag -
1. Regarding key size - Yeah, as you know 8mm & 2mm is essentially the same size as the 5/16 & 3/32. (7.9mm & 2.38mm or so) Our designer chose that shaft size somewhat arbitrarily - 1/4 diameter would probably also work.
The limiting factor here strengthwise is the yield strength of the keyway in the plastic gear - (weakest component in the plastic gear- steel key- steel shaft load path)
You need to find average pressure on the plastic keyway surface at max motor torque = stall torque. It needs to be below the yield strength of the plastic. And - We usually use a safety factor of 3 or 4 to account for impact loads etc.
You can analyze this pretty easily -
I would get the motor stall torque & gear reduction - find stall torque on the shaft (it's going to be 3-4 x motor torque)- and then divide by shaft radius to the midpoint of the keyway to get force acting there.
Divide by keyway area, and see if it's above the yield strength of the plastic.
[If you don't have a strength, many engineering plastics are around 3000 psi yield.] You may have been here already.

2. Re: synchronizing/reducing drill & fp motor speeds to a common shaft - see my lengthy description in last post, it should be clear. I can always email you some more photos of tranny from different angles. We have them around there in some directory.

3. I think one key in an efficient design at this first step of reduction is to keep your bearing friction down, (ie use ball bearings & smallest possible dia.) these first shafts off the motors are spinning 1000-3000 rpm, and thats a lot of rubbing goin on if you use large diameters or bushings. Still, bushings are cheap and that's worth a lot.

Jesse.