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RHD 02/11 Maintenance Section 5-18
© 2011 Alamo Group Inc.
MAINTENANCE
General Information on Flail Mower Vibration
FLAIL MOWERS - INFORMATION BULLETIN NO. 5
Excessive Vibration
Vibration is a MONSTER
and if allowed to continue unchecked, can cause the complete break-up of a machine
in a very short time.
A properly operated Flail Mower will produce very little vibration. If vibration does occur, the mower should be
stopped and the trouble corrected, as warranty does not cover failures resulting from continued
operation after something happens to cause the mower to vibrate excessively.
Vibration is caused by a rotating part which is out of balance. This could be the cutter shaft or, on those models
having a universal slip joint assembly drive shaft, from the drive shaft.
If the two extreme ends of the drive shaft, (the splined yokes which fit on the tractor power take off shaft and
on the mower gear box driven shaft), are not parallel, vibration will be evident. This is because of the
mechanics of a universal drive wherein the rotational speed thru a universal joint is not uniform. If the ends of a
universal drive shaft are parallel, the uneven rotation effect is cancelled out and the machine being driven will
be driven at a uniform rate of speed. However, if the ends are not parallel, the uneven rotation effect may be
multiplied, resulting in a speeding up and slowing down of the entire mower system (gear box, extension shaft,
pulleys, belt, and cutter shaft) each revolution. This effect can be readily noticed if the mower, while running, is
raised to an extreme height on the tractor lift.
For smoothest operation, the tilt of the mower should be adjusted (on a 3-point hitch, by adjusting the length of
the top link between the tractor and the mower) so that the ends of the drive shaft are parallel when the mower
is down in operating position. Raising the mower while running, to extreme heights on the lift should be
avoided.
To check where the vibration is originating, first be sure the ends of the universal drive shaft are parallel. Then,
if vibration is still present, remove the drive belt and run the mower. If vibration is still present, it most likely is
coming from and caused by a bent drive shaft. If so, the shaft should be replaced or straightened. If the
vibration is gone - the drive portion of the mower is running smoothly - then the vibration is probably coming
from the cutter shaft (it could be a pulley out of balance or a faulty belt with a varying cross section, but this is
not likely).
The cutter shaft will run out of balance if:
1. Wire, rope, string, rags, etc., around the cutter shaft are holding the knives in a folded back position.
Remove such foreign objects and be sure all knives are free to swing to the extended position.
2. Knives are missing or broken. Replace missing or broken knives.
3. Knife hanger lugs are broken off shaft. This condition usually results from continued operation in contact
with large hard objects where the knife hanger lugs themselves are hitting the hard objects. Lugs should
be replaced using arc welding equipment so as not to direct too much heat in one area of the shaft.
4. Cutter shaft ball bearings are worn enough to have radial “play” in them. Replace ball bearing and other
worn parts to eliminate “play”.
5. Shaft is bent. The shafts are straight within a couple of thousandths of an inch when they are made. It is
possible that they will become bent in use, such as if the shaft is hooked on a stump while the tractor is
moving forward at good speed.
6. Shafts are operated at improper speed. Rotating shafts have what is known as a natural period of
vibration. The same is true of the cutter shafts on the Flail Mowers.