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PTO Maintenance

Nov 23rd 2023

PTO Maintenance

Tub Grinder – PTO (Dry Clutch) Maintenance and Operation Tips

(Warning) improper adjustment of the mechanical PTO (clutch) can cause catastrophic failure of the PTO the first time you feed material to the grinder.  Before operating the PTO on your tub grinder, be sure you’ve read and understand your manufacturer’s operations manual.

 The following supplemental instructions provided here are a short list of tips you’ll want to use daily.
Twin Disc and Rockford are the most common dry clutches (industrial PTOs) found in tub grinders.  There are a few size variations, 14” and 18” models the most common, and based on the engine and horsepower of the grinder.  These clutches are relatively simple and fairly easy to repair, however, they are vulnerable to costly damage from operator error.  The following is a list of tips to get the most life out of your PTO and save some big $.
  1. Check adjustment daily and adjust as required (see maintenance for proper adjustment).
  2. Properly grease the PTO bearings, usually a small amount daily.
  3. (Warning) Do not engage the clutch with material blocking hammermill rotation
  4. Engage the clutch at low idle (less than 1000 RPM’s)
  5. Using “Bumping” technique described below to engage the PTO and start the hammermill
  6. Do not drop a large amount of material directly onto the hammermill when the tub is empty, large amounts are normally okay once the machine has material in the tub to act as a cushion
  7. Allow the tub to run empty before disengaging the PTO

Clutch Maintenance / Adjustments

  1. Typically the clutch should require between 150-200 lbs of force to engage.  Said more plainly, most industrial duty PTO’s require that a man use his full body weight pushing on a long handle.  When engaging the clutch takes less than roughly 150lbs of force to engage, or when you can engage it with only your arm strength and no body weight, the clutch is likely not adjusted properly and is likely to slip during operation.  Checking and/or adjusting a dry clutch is very easy and should be part of your daily maintenance.   Check the inspection plate on your PTO for the proper amount of force required, and note that it will be in foot pounds, not inch pounds.  Failure to keep your clutch properly adjusted can result in slippage, overheating, increased wear and warping of the clutch’s friction plates.  “Remember“ improper adjustment of the PTO can cause catastrophic failure the first time you feed material into the hammermill.

If your machine has a power-assist unit to engage the PTO such as an air or hydraulic cylinder, it will be necessary to disengage the power-assist and test for proper adjustment manually. Additionally, if the power-assist is out of adjustment and is constantly putting pressure on the PTO in either the engage or disengage direction it will cause excessive wear of your PTO's throw-out yoke and bearing.

  1. Grease your clutch according to the manufacturer’s specifications.  Most clutch housings have an external grease port that you can use as required.  If you have the external grease fitting it is also important to check the internal grease line inside the clutch housing to make sure it is intact and functioning properly.  The internal grease line often fails when a clutch overheats and many operators fail to repair/replace it.  This will result in improper greasing and clutch failure.

Engaging the PTO

  1. Only engage the PTO when the hammermill is completely free and clear of material and debris.  Not only is it a safety risk to engage the PTO with material in or around hammermill, but it will also greatly reduce the life of your clutch and possibly result in immediate clutch failure. 
  2. Only engage the clutch at low idle ; less than 1000 RPM’s.  For most machines this is around 800 RPM’s.
  3. Engage the clutch using a technique called “Bumping”.

Move the lever partly to the engaged position, then quickly back.  You will hear the engine rpm’s drop as it works to begin turning the hammermill.  Bumping does not require a significant amount of force.  On many tubs where the driveline is exposed, you will see it begin to turn as the hammermill builds momentum.  Do this 2 or 3 more times in quick succession, within roughly 10 seconds, then engage the clutch fully; be careful not to let the engine RPM’s drop too low such that the engine dies.  Bumping allows the hammermill to overcome inertia, start rotating, and gain momentum before the engine must pull the full load of the hammermill.  This method of engaging the clutch will greatly extend clutch and engine life.
Feeding the Tub Grinder

  1. Avoid dropping material directly onto the hammermill when you first start feeding the tub grinder.  The impact of the material hitting the hammermill can cause the discs to slip resulting in overheating and possibly clutch failure. 
  2. Begin feeding the tub grinder by piling material on the tub floor opposite of the hammermill.  Once you get a small pile, engage the tub into slow-forward.  Continue to feed material until the tub is full and the hammermill is buried.  Keep the tub full and the hammermill covered until you are finished grinding as this will dramatically cut down on the danger of thrown debris.

Shutdown (for normal operation) Make sure that any emergency stop buttons on the machine function properly for quick shutdown.

  1. Allow the tub to grind all of the material in the tub and spin freely.  If the tub is shut down with material in the tub, it will have to be cleared before the clutch and hammermill can be engaged again. 
  2. Disengage the clutch and allow the hammermill to spin freely to a stop.  Do not use the clutch to try and slow the hammermill.   Using the engine to slow the hammermill will greatly reduce engine and clutch life.

EarthSaver Equipment offers parts and complete assemblies for most tub grinder dry clutches.  Call for a quote (866) 227-2244, or e-mail us at