mower rotor balance

[AJH]

Probably a bit OTT for d-i-y but in the short time each day between sitting stationary on the M25 I've been using a large mower. It has a horizontal shaft which revs at 1000 rpm and has a total of 60 fixed teeth in 6 rows, within a width of 2.3 metres.

As the teeth wear unevenly, and are expensive, I replace them one at a time but this causes the rotor to go out of balance. I can try to remedy this by swapping part worn and new teeth around the rotor but I would like to better dynamically balance the rotor by having an insight into where the heavy spots are.

I'm guessing a dynamic wheel balancing machine works by sensing the out of balance force, but how?

Would an accelerometer clamped to the mower, whilst it's suspended off the ground, and linked to fire a strobe be a feasible, cheap, means of identifying where to change a light tooth for a heavy one, or vice versa.

AJH

[meow2222]

The simple method is to spin the item up while hung from a non-rigid support so that it can wobble easily. Offer a crayon up very close so it barely touches, and hold steady. The crayon will mark the most sticky out bit. Its impossible to remove all imbalance this way, but most yes, and its simple.

NT

[Tim Lamb]

In message , [e-mail] writes

I think issues of this sort are avoided by replacing worn cutters in pairs if the mounts are divisible by two.... or threes etc.

Part worn parts can be weighed and fitted as matched sets.

Usually the removable parts are not a large part of the rotating mass. The knives on a combine straw chopper are a tiny mass compared with the shaft and mounts.

regards

[AJH]

It can be suspended but only from the rear mount of the tractor,

A trifle dangerous though and in this case its not only the protruding bit that wears. To put a bit of perspective on this the rotor weighs over 1000kg, I just weighed a new tooth at 1.611kg and a half worn one at 1.318kg. I guess a worn out one drops to 1kg.

Yes Tim that's right but as I said they're rather expensive.

Yes I'll have to resort to that if I cannot find a better solution but there is the complication that the rotor itself wears, possibly unevenly.

Yes but its still a large out of balance mass and that's what causes the problem.

I rushed back in sunshine and no traffic expecting a host of techie fixes :-(

Now what I was thinking was a simple electronic device just to show which was the heaviest part of the rotor. What about a something that could be clamped to the top of the mower consisting of a small magnet attached a horizontal stainless steel ruler, such that its resonance frequency was well away from the rotor rpm or any of its harmonics. Just off to the side of the magnet a pick up from a cassette deck. As the rotor revolves the out of balance force will force the mower deck down as the heavy part of the rotor reaches the bottom. The magnet will resist this downward acceleration and so the pickup will move down. Could the signal from the pickup then be used to trigger a strobe light which would illuminate the bottom, heaviest , part of the rotor?

AJH

[meow2222]

I think it would really help if you gave us a pic to look at. Its hard to offer any better than general ideas blind. Some info on the problems the imbalance is causing in your case may also help.

NT

[Tim Lamb]

In message , [e-mail] writes

AIUI this a heavy duty, full width, rear mounted *mulcher* attached to an agricultural tractor.

Ground contact is likely to be unintentional so the tractor has to absorb any vibration. Probably shaking the fillings out of his teeth:-)

Bearing wear may be a secondary issue.

regards

[fred]

In article , AJH writes

A long time ago I saw an idea for a self balancing washing machine drum which used a circular race part filled with large ball bearings mounted in line with the drive pulley. The idea was that the balls migrated to the light side and balanced the rotor. From memory the race was just smaller than the drive pulley and the balls were 10-12mm dia. I'm not sure whether you would want the ball movement to be damped (say with oil) or undamped.

You'd need a machine shop to make a perfectly circular race but I thought I'd mention it in case it helps.

[AJH]

I've put up a couple I took with my cell phone camera this morning. A bit blurred I'm afraid.

've left them full sized.

That's right

In fact ground contact is deliberate to bust out the roots, when in the ground there is little vibration, it's most noticeable when raised at the end of runs.

Not it's no worse than driving my Land Rover to the site each day!

Wear and tear on the machines is more of an issue.

I went round the rotor today swapping out likely "mis matched" pairs with new cutters, it worked well. I'll see if I can match the part worn ones on a kitchen scale, following your suggestion, and then reuse them.

I'd still like to see the possibilities of an in situ balancing device.

AJH

[meow2222]

I'm not familiar with the machinery, but if I'm guessing right the teeth dont slide past a bar to cut, they just dig into the ground. If this is so then a possible alternative to balancing may be to add some small travel suspension to the main bearings, permitting movement and thereby much reducing the forces on the bearings.

NT

[AJH]

There is a rudimentary shear bar but it has a huge clearance, passed a

200mm long bit of railway track through it yesterday. The machine just relies on momentum to smash anything in its path.

I doubt it, everything needs to be very rigid. There's no doubt the rotor must be kept in balance and the simplest way is to change teeth in pairs, I was just looking for a simple device to show up which part of the rotor was out of balance.

The boss sent up a smaller version, hydraulically driven on a skid steer unit to mow perpendicular to the rows of trees. Damn thing is so fast when revved up I can hardy feather the joystick back far enough to keep the speed down to 2kmh. Now I'm looking for some small coil springs, 10mm internal clearance 7mm closed height to put some stiffness and limit on the joystick travel, any suggestions for sources?

AJH