3-speed gearbox - grinding

[Mad Ax 9313]

I've probably just done something real stupid, but I hooked up the MFU to my part-built MAN TGX at lunchtime and the gearbox was making a bad grinding sound when running, and I can feel a vibration thru the chassis.

It shifts smoothly and the roadwheels turn with the motor, the grinding increases in pitch as I go up thru the gears so it's almost certainly something on the output side - I've probably just put a gear in the wrong way round or maybe used the wrong screw assembling the case so the teeth are touching something they shouldn't, but thought I'd throw this up in advance just in case anyone has experienced this before.

I wasn't going to run the truck until I'd finished the shell but I was in the process of getting the wiring nice and tidy when I thought I'd best give it a test-run.  I'm really glad I did, although there's still a lot of disassembly to do to get the transmission out 

[Tamiyabigstuff 832]

Check the motor pinion is correctly fitted and that the pinion grub screw on the motor shaft is not touching the plastic gear... been there, done that.

I always test run these gearboxes before putting the casings on and installing.

[MadInventor 3884]

1 hour ago, Tamiyabigstuff said:

Check the motor pinion is correctly fitted and that the pinion grub screw on the motor shaft is not touching the plastic gear... been there, done that.

I always test run these gearboxes before putting the casings on and installing.

Yeah, me too, this was my first thought on it.

It might also be the two 2mm screws used to hold in the bearing holder at the top on the rear aluminium plate, easy to put in screws that are too long and touch the gears.

 

[Mad Ax 9313]

Thanks guys, it was exactly this - I'd thought it was something on the output shaft but the grinding was probably compounded by the increase in general transmission noise as the axle speed increased.  The pinion grub was catching the plastic spur.

I'm using a new Carson Poison truck motor to keep speeds down, never used one before and didn't run it before I'd fitted it.  It's a seriously noisy motor, I span it up while not connected to the gearbox and it sounded like a bushing was worn, although it's brand new.  For a moment I thought I'd have to junk it and fit something else (I'm in a hurry to get the truck finished for an event in 2 weeks and didn't want to have to wait for another to be delivered).  After re-fitting and testing it was still seriously noisy until I slackened the gear mesh off a little, it's almost like the motor was transmitting its vibration right through the gears and amplifying it inside the casing.  Weird.  With the mesh backed off a little it's much quieter.

Having to pull the transmission out wasn't a bad thing as it gave me the opportunity to route all the MFU wiring down into the chassis rails out of the way.  Half an hour with a bunch of zipties and it's looking very neat and tidy now.  Tonight will be spent prepping all the body components for paint, a job that I have never relished...

[GooneyBird 938]

I've built a High Lift with the same gear box. They can be loud, but a bit of chain lube on the gears goes a long way. (Grease actually increases the resistance in the gear box, making it quite loud) Make sure you don't replace the bushing in the sliding gear, as a ball bearing cannot cope with the sliding motion across the shaft. Been there, done that.

Also, try to use rubber grommets when routing cables through the chassis. Again, speaking from experience. The vibrations will cause any hole or sharp edge to eat away the insulation. Again, speaking from experience.....

[Mad Ax 9313]

Thanks Gooney.  I noted in the manual it said not to use a ball bearing in the gearbox.  In fact the bearing kit that I ordered for my TGX seemed to be missing a few bearings, although I remembered to use a bushing in this particular place   Vibration unit and MFU speaker are currently masking the sound of the gearbox

I got all my rear lights, MFU cables and hitch switch all routed real nice and looking tidy, noting the manual said I could have aux lights or back lights in the rear.  It was only after I'd neatly routed and bound everything with zipties that I realised I could actually fit both aux lights and back lights in the light buckets, so I guess I'll have to try to get this one extra wire to follow the same route and add extra zip ties.  Bit of a pain!

The next mammoth task is the sheer huge volume of body painting I have to do before I can do any more assembly...  The next couple of weeks could be quite hectic in the paint booth!