3771 Chassis

[kmcclary]

A VERY important thing to remember about Mecanum wheels is that, UNLIKE standard wheels (or even Omniwheels!), for every forward or reverse pound of thrust you attempt to derive from a Mec Wheel, you ALSO get a pound of SIDE FORCE transmitted ALONG THE AXIS of the shaft, which MUST be transferred to the frame WITHOUT BINDING.

This is because the 45 degree rollers shafts "vector" the forces you apply equally to the side as well as forward & back..

To handle these extreme side forces, it means you need to add THRUST BEARINGS on both ends of each shaft. This insures the wheels "push" on the chassis and do not "bind" TO the chassis from friction whenever you drive them. (IOW, adding TBs GREATLY increase efficiency.)

We've had very good luck with VXB Bearings, for our Mecanum Drive thrust bearings:
... www.vxb.com
Search for "Thrust", and find some that fit your shafts. (They ship same day, too... )

BTW... If anyone has other bearing sources they like, please also post them here! Thanks!

Thrust Bearings come as what looks like a triple pair stack... A pair of flat or grooved "washers", sandwiching a ring containing a circular set of either ball bearings or radially mounted needle bearings. Buy enough sets in your shaft size for each end of every wheel shaft (plus some spares...).

Simply add one tri-stack to each side of each wheel. Make sure if the "washers" are grooved that the grooves face the bearing ring.
(Personally I prefer stacking them immediately on either side of the wheel itself vs between the spacer and the frame, because this also eliminates all "spacer spin drag" on your shafts as well.)

...Easy Breezy...

BTW, TBs help with NON-mecanum drives too, by (a) removing your side force wheel-to-chassis friction whenever someone pushes you sideways! (At those times, every little advantage to help you escape is appreciated... ), and (b) to keep wheels from binding because someone went crazy in the pit tightening the shaft bolt down too much and the spacers pinched the wheel.

Note that you STILL need REGULAR bearings too, for allowing the wheel and sprocket assembly to spin on the shaft! The Thrust Bearings just "surround" your wheel-&-sprocket assemblies, to help them transmit the SIDE forces to the frame without binding the wheel.

If you forgot about them during initial assembly, no sweat. They're easy to add later in the pit when they come in simply by trimming the right amount of shaft spacer length and stacking them in around each Mecanum Wheel.


One OTHER thing... With Mecanum Drives, you also need to make sure all four wheels are on the ground at ALL times, or it gets wonky... With a flat field like this year's game (no "ramps"), "frame flex" is normally sufficient to eliminate the need for suspending a pair of wheels on a pivoting subframe,

BUT, if you make your frame TOO STIFF, you MAY encounter a touch of drive vectoring problems should one wheel lift up too much and loose traction. Watch for that.

IF it becomes a SERIOUS problem, there are methods to later add just a "wee bit" of spring action to one end's pair of wheels to cure it. The symptom to watch for will be the bot spinning or vectoring in directions you didn't intend it to go, especially with strong moves that tilt the bot slightly, removing pressure from at least one wheel.

Just let me know if this happens to you, and I can point you toward some solutions for it.

Good luck! I hope this helps...

- Keith
(Who also happens to be a List Dad for an Omni and Mecanum Drive elist...)