775 Pro Coupling
 

Has anyone come across a 3mm press fit coupling for the 775 Pro?

I am trying to adapt the motor in a high RPM application directly to a pulley.
The GT2 pulley that WCP offers has a large cantilever due to the fact that the 775 pro has such a short output shaft. I would like to find a coupling that does not use a set screw (in fear of it coming loose) to a direct drive a gt2 or like pulley. If anyone knows of something that gets you from the motor too 1/4" or 3/8" diameter shaft and can be used in a high RPM application please let me know.

Thank You :)

Re: 775 Pro Coupling
 

Sure you can't use a 1:1 VersaPlanetary?

Re: 775 Pro Coupling
 

Yea... I would use a versaplanetary 1:1 or even 3:1 these guys are faster then you think.

IMO directly mounting to those shafts is a bad idea.

Re: 775 Pro Coupling
 

The versa planetary would definitely work, however you must use a set screw to install it. I may be over-cautious, but I am fearing set screws this year because of the nature of this game and I don't want anything to come loose, especially in a high RPM application such as a shooter wheel.

Re: 775 Pro Coupling
 

We used a set screw+A shaft key. It doesnt directly key the motor shaft but it does help. You could also cut a small slot in the axle with a dremmel, or lathe and connect it to the adapter with a DIY shaft key. The adapter has a gap so its not an issue.

Re: 775 Pro Coupling
 

Think about it this way. No matter what gearing the output of a VP is spinning at, the input will be spinning at the free speed of a motor. If that coupling has held on at high speeds to motors for years, why would it fail now? I'm not saying set screws in general are the best idea, but the VP design is tried and true. The VP will also provide a much better supported output shaft than you would have otherwise.

This is a horrible idea. How do you cut a slot in a 3mm wide shaft? How big would the slot even be? Maybe 1mm?

Re: 775 Pro Coupling
 

Looking to do something like this.

Re: 775 Pro Coupling
 

Re: 775 Pro Coupling
 

If your looking to do that, then its the same as buying the GT2 pulley here:

http://www.wcproducts.net/gt2-timing-pulleys-belts

Re: 775 Pro Coupling
 

What are the recommended installation instructions (ie spacing from motor) for the WCP pulley?

I am interested to see it installed and running, I have a fear about the cantilever off of the motor output shaft because there is so little engagement. There is very little info about it on the website.

Re: 775 Pro Coupling
 

Your not going to run into an issue with shaft stick out, it is the same as the Banebot 775. 254 & 971 run the 12T pulley on their stuff, you can check out CAD from FRC971 on their site.

You can also email support@wcproducts.net for more info/concerns.

Re: 775 Pro Coupling
 

We were also concerned that 775 motors might not handle side loads well.

We've had good results using 3/8 hex shaft stock (217-3310) to fabricate 5mm bore press-fit shaft adapters for the 775pro. Here is our method: (lathe)

-- rough cut stock ~1 inch long using chop saw or band saw
-- face both ends to give 7/8 inch finished length and break edges
-- center drill
-- drill through #21 (0.159 inch dia)
-- drill about halfway through #9 (0.196 inch dia) <-- this step gives a press fit hole diameter for the 5 mm motor shaft
-- tap the smaller end 10-32
-- using arbor press, align squarely and press motor into large end of adapter

(Note: arbor press ram should only touch the back shaft, not the motor body)

A 3/8" hex bore gear on the adapted motor shaft drives a larger gear on a 1/2" hex output shaft. Both shafts are supported by bearings in a gearbox housing, which we make on a mill using 1x3 rectangular aluminum tube with 1/8 inch wall thickness. The output shaft bearings are installed flanges-in with a 1/8 inch plastic spacer between the gear and one bearing. The motor face mounts to one side of the tube* secured by two M4x10 button head screws, then a spacer and the shaft gear go on, and a 3/8" hex bearing (217-2735) goes in flange-out. A 10-32 x 0.5 inch button head screw and washer keep the bearing in.

Free current draw with this method is less than we saw testing with a Versaplanetary, and the gears are easier to keep lubed. It is also easier to integrate this gearbox into our shooter because the output shaft is also a shooter wheel axle, supported by another bearing on the opposite side. Another benefit is that we can select any of several gear ratios that use the same total number of teeth. That includes options smaller than 3:1, which is lowest ratio available using Versaplanetaries.

---------
*Vent holes in the tube wall, aligned with those in the motor face, are highly recommended.

Re: 775 Pro Coupling
 

Rich, Thank you for the input! We are leaning toward that method. I would like to see your setup. Do you happen to have a finished product?

Re: 775 Pro Coupling
 

Is anything on a FIRST robot ever finished?

The setup we are testing now is pictured here. Please let us know what you think, and offer suggestions to improve.

Re: 775 Pro Coupling
 

To echo what RC is saying here, we've been doing this for years, with RS550's, 775's, and are planning on doing it with 775 Pro's this year. RC added that part to his inventory based on feedback from us. We press it on to the 775 with the short flange 0.020" from the motor face to reduce the cantilever. We've never had an issue. (Last year's bot had 7 per robot 775's with pinions pressed on as described. We had 0 failures.) With bearings in the new 775 Pro, this should be even less of an issue.

Re: 775 Pro Coupling
 

Do you guys have a source for other size gt2 pulleys? I can buy the pinion from WCP, but where do get your others from? (I was hoping mcmaster would sell them).

Also based on it's size it looks like gt2 is close to MXL, any experience in the difference between the two?

Re: 775 Pro Coupling
 

Re: 775 Pro Coupling
 

We buy and broach pulleys from SDP-SI. It is a lot easier only having to source pulleys and not pinions. We previously had issues reliably sourcing pinions. It would be nice to be able to buy hex broached pulleys, but I don't know of a source for that yet.

MXL may be close in pitch size to GT2, but GT2 has significantly higher power ratings than MXL. I don't remember the number (you can find them in the Gates manual if you want), but it is close to an order of magnitude of difference. I doubt that MXL would handle the motor torques, but you should check the numbers yourself.

We did the same thing for one of our prototypes. Pressing a short section of 3/8" hex shaft onto the motor has worked great. We hex broached some timing pulleys and secured them with a shaft collar. Being able to quickly swap out pulleys or gears on the motor is really helpful. We used a 0.195" reamer for the hex shaft. This is a little on the tight side, but it goes.

Re: 775 Pro Coupling
 

Can you point me to a C-C distance calculator that works with the GT2 3mm 12 tooth pinions that WCP sells? The one at SDP-SI only goes down to 16 tooth.

I would like to use this pinion to drive a 44 or 45 tooth pulley from SDP-SI. Will exact C-C set up work, or will I need a side tension system to keep six teeth of the pinion engaged to the belt?

Re: 775 Pro Coupling
 

In the past I've just emulated the Versa strategy.

I used the VexPro "c" collar from the Versa and just reamed the pulley to 8mm then applied clamping force from a set screw.

Re: 775 Pro Coupling
 

It's actually pretty easy to do in Solidworks. Draw out the belt as a bunch of arcs with a diameter of (3mm * num_teeth / pi) and straight lines. Use the Path Dimension feature (It's in the drop-down menu in the sketch menu under the Smart Dimension tool) and select all the lines. Set the path length to (3mm * num_belt_teeth). Done!

For short belt runs, you might want to remove a couple thou. We tend to just run exact c-c and move on. For longer ones or more critical ones, we add a tensioner.

Re: 775 Pro Coupling
 

Thanks for the CAD trick -- I will get one of the students to try it!

An equivalent old-school method uses pitch diameter circles and tangent lines connecting them to calculate the wrap angle on each pulley, and then get center distance from the resulting two arcs and two lines. One such calculator can be found here: http://www.gizmology.net/pulleysbelts.htm

Using that one I got 4.773 inch C-C for a 110 tooth belt connecting a 12 tooth pinion to a 45 tooth pulley at 3mm tooth pitch. Will check that against the CAD result before we cut holes in metal. Measure twice ...

Re: 775 Pro Coupling
 

You can change the type of pulley to "timing pulley stocks" to get down as low as 6t. You also have to go to Metric instead of Inch.

EDIT: I got the same result but with only 5 teeth in engagement.

Re: 775 Pro Coupling
 

Our favorite reduction is a 56 : 12 with a 67 tooth belt. I think we might have had 6 of them last year. I love GT2 since there is such a safety factor that it isn't a huge issue if you don't get everything else perfect. (6 teeth of engagement is 180 degrees. I try to target at least 120.)

Re: 775 Pro Coupling
 

We were going to do that. It was all CADded and everything. Then I looked at the load rating of 3mm GT3 belts. They were only rated to 10000 RPM on the faster sprocket (unless I'm reading it wrong). So attaching it to the 775pro with a free speed of >18000 RPM scared me.

Re: 775 Pro Coupling
 

We're looking at how to get a 19t 32dp pinion onto our 775Pro shafts for our shooter. My students effectively designed a clone to the AndyMark Cim-Sim, stealing the shaft, cluster gear, and bearings and using a 19t gear from bane bots that was thought to be appropriate. Long story short, that gear has a 5mm bore, but is also 40dp (we think). As of right now the plan is to modify 19t 32dp gears from AM to fit the 775Pro.

My dilemma this morning was related to fixturing the gear, but also whether to use a #10 drill then ream the bore to .1955" or .1960". This gives you either .0007" or .0012" interference. At this size a proper press fit is .0002" to .0014" interference. If I don't have to spend $40 on tooling it wouldn't hurt my feelings though. My only concern is that a slipping pinion might be pretty difficult to diagnose and could be a pretty severe failure mode in a competition environment.

Re: 775 Pro Coupling
 

I agree with all your points regarding proper interference fit, and reaming is clearly a better method. I think we just got lucky with the #9 drill. Over the past 30 years, I have press fit a lot of motor shafts from 2mm through 17mm diameter and have developed a feel for the forces involved, so I am pretty confident of the results we got on a few motors. I would not use the same method in serial production.

To your question: I'd go with the 0.1955" ream. We will probably do the same when I need to make more components.

Re: 775 Pro Coupling
 

Thanks for the input. I'm afraid in the process of removing the improper pinions we may have damaged our motor set, so my intent is to use the .196" drill for this first set of questionable motors to be used in program testing and development, then put in our order for a competition set and back ups for those at which point we may go ahead and get the reamer and take that time.

For anyone wondering, the reamer is McMaster part number 8803A32. You have to specify that you want the .1955" from the range associated with that part number.