Better Gas Winch

Last crawled date: 1 year, 11 months ago

After building Gas Winch 1.0, I realized that I had to do better.

Goals:
*** Pull on the string to re-load winch (bearings are sized for gas spring compression loads which are much higher than output loads).

***Less fussy latch/release mechanism

***Cleaner outer appearance. Keep the fancy bits inside, provide fewer pinch points for fingers and such.

***Servo release. Especially attractive now that FIRST has taken servo power off the RoboRIO power supply :)

***Modular Design - more packaging options and more travel options to fit more end of match winching needs.


Regarding Modular Design:
Each Gas Spring unit is designed to fit inside a 2"X2"X1/16" wall square tube (available from OnlineMetals.com).

Currently you get 48" of winch travel with 275lbs of force in a 2"X4"X32" package.

You could easily convert this to use the 16" stroke gas spring and get by with a single unit, assuming you can live with the 8 inches longer overall height and only 32" of winch travel at up to 275lbs of force. This package would be 2"X2"X40" << perhaps perfect for a boom that carries the hook that reaches the bar??? Another advantage of this configuration is that it would weigh perhaps 60-70% of the dual 12" version shown in the design.

Alternatively, you could also use 8" stroke gas spring and stack (3) of these units to get 8 inches shorter over all length and still have 48" of winch travel at up to 275lbs of force. Note this option has the down side of weighting ~50% more than the dual 12" configuration shown in the model. This package would be 2"X6"X24" << perhaps this would fit better in your robot???

A variation on this option would be to have the (3) 2"X2"X24" tubes arranged in an L shape each leg of the L being only 4", for a bounding box of 2"X4"X24" << again, maybe this is the perfect fit for your robot???

I don't recommend this but you may decide to build these 2"X2"X1/16" wall tubes into your robot chassis. In general, this seems like a crazy idea but in some cases it may be crazy like a fox.

YMMV. If you try this and end up sad, don't say I didn't warn you...

Regarding Servo Release:
Servo release SHOULD be okay if bearing preformance is what I expect it to be. I need 2 mini-servos to release each gas spring but even with cheap Hitec servos I expect to have over 100% safety margin on unlatch/relatch torque.

Which reminds me, you need to power the servo to "relatch" once you have compressed the gas springs. FWIW, the net unlatch/relatch torque is approximatly the same. Unlatch has a larger moment arm but only 550lbs force. Relatching has 825lbs force but a smaller moment arm.

Finally on servos, I have used the very cheap HS-45HB AGTT Feather Servo $17 on Amazon - Hitec spec: 1in-lbs stall torque at 6V. There are other servos in similar sizes that have 2-4X the torque. Will investigate these should the margins start to worry me.

Servo Spec Here:
http://hitecrcd.com/products/servos/micro-and-mini-servos/analog-micro-and-mini-servos/hs-45hb-agtt-feather-servo/product

FWIW, I assumed a "coefficient of friction" for the ball bearing and the thrust bearing of 0.005 -- which seems reasonable based on web searching.

Regarding Manufacturing:
Many parts are designed to be 3D Printed.

There are some complex machined parts (the bottom block and the top pulley housing).

Bottom block is probably aluminum but will require a lot of machining (nothing fancy but there is a lot of metal to remove to provide the pivot for the bottom pulley).

Top Pulley Housing is probably Aluminum but may go steel. Still thinking.

Latch wedges are definitely steel. Maybe even hardened on the end surface to keep things from mushrooming over and raising unlatch forces.

Regarding Bearing:
Latch Bearing are 5905K210_STL NEEDLE-ROLLER BRNG
470lbs static load rating. 550 lbs max load share by 2 bearings. They never see gas spring's reloading force. Steel tube around the bearing keeps the outer race from deforming.

Thrust Bearings are 5909K320_THRUST NEEDLE-ROLLER BRNG
McMaster doesn't quote a static load spec but dynamic load rating is over 2000lbs. Should be fine even for 825lbs compression force for the gas spring.

Pulley Bearings are 60355K505_BALL BEARING
This one is kind of dicey on the bottom pulley. The rated static load is 535lbs. This does not really support the 825lbs compression load of the gas spring. BUT... I am hopeful that since the dynamic load is 1145lbs, and specs are often somewhat conservative and FIRST FRC is a low cycle type application, we can get through a season without incident. Not a great plan but it is all I have got unless I can find another bearing that fits. For Example, I can almost make 60355K506 fit even though it is a little wider (0.281" vs 0.250"). It has a 730lbs static rating. (Done. Update 2017-01-06)

The good news is the upper pulley uses 2 bearings so no worries at all on that front.

Not yet in the design:

Not shown are 10-24 countersunk flathead screws that are running up and down the outside of the 2"X2" tube. The idea is that these flush screws will allow for a "truck" to slide on the outside of the tube to provide a well defined motion for the hook or whatever you use to lift the robot. (Some screws shown. Update 2017-01-06)

Also not shown are the gear teeth on the servo gear and the unlatch tube. They are "standard" 32DP 20 Degree Pressure Angle gears (10T and 48T respectively). The gears on modeled as cylinders with the standard OD. If you look a the models closely, I have also indicated the Root Diameter and the so called "Pitch Circle" with a very small groove in the edge of the part. I will be updated the CAD to have the true gear profiles soon. (Done. Update 2017-01-06)

Comments welcome.

UPDATE 2017-01-06:
***Added actual gear teeth to servo gear (but spline is still faked)
***Added actual gear teeth to unlatch tube
***Reduced gap between latch roller and latch plate
***Added some 10-24 flat head screws (but not all)
***Changed Bottom Block to have in "integral axle" after talking with CNC source -- makes the part much much better in function (in exchange for making it harder to machine)
***Changed to larger bottom pulley bearing. Still not 100% within spec for compressing the gas spring loads of 825 lbs. but it comes close 735lbs

Still to do:
***Put in remaining screws
***CAD actual Servo Spline and Servo Screw
***Make 8" travel version
***Make 16" travel version
***Make a new ZIP file and upload it


Dr. Joe J.

P.S. Zip file is a SolidWorks PackAndGo (professional edition -- sorry I don't have educational version available to me).