Getting to Know your Kubota’s Clutch



If you are old enough to drive then you know the basic idea behind a clutch – it is a component of transmission that is designed to allow the engagement or disengagement of the engine to the gearbox. Without a method of temporarily disconnecting these two assemblies you would not be able to shift gears or change direction of travel – both very handy features of your Kubota tractor.

Depending on the model of Kubota tractor you own it will have either a dry clutch or a wet clutch.

Wet Clutch
Certain Kubota tractors use a multiple plate wet disc clutch system that is in-place of the dry clutch. This hydraulic wet clutch offers exceptional durability and a much longer operating life compared to a single dry clutch disc. The wet clutch has a steady stream of filtered hydraulic oil that keeps the clutch discs and plates cool, reducing friction for smoother operation. A mechanically actuated shuttle valve controls the directional flow of oil, allowing the operator to shift between forward and reverse gears without clutching. Kubota’s L2550 or L2650 are two examples that employ a wet clutch.

This image is an example of a wet clutch pack. We see two gear packs - one forward and one reverse. The entire unit is bathed in a lubricating oil.

This image is an example of a wet clutch pack. We see two gear packs - one forward and one reverse. The entire unit is bathed in a lubricating oil.

Dry Clutch
Dry clutch Kubota tractors are ones that operate with a dry asbestos friction disc material. The friction material is pressed tightly against the tractor’s flywheel using springs. They do not last as long as their wet clutch counterparts, but they are cheaper to repair or replace. It is also possible to rebuild a dry clutch and its common to have the disc resurfaced. Most of Kubota’s model lineup use dry clutches. B5100, B6100, B7100, B5200, B6200, B7200, L175, L185, L1801 and so on – all examples of dry clutch Kubota tractors.

A typical dry clutch with asbestos friction material. The springs relieve load shock and help smooth clutching action.

A typical dry clutch with asbestos friction material. The springs relieve load shock and help smooth clutching action.

Kubota’s “Ever Clutch”
The L2650 is an example Kubota that was available with either a wet or dry clutch. L2650s with an “Ever Clutch” decal were fitted with the longer lasting wet clutch system – hence the “Ever”. Other Kubotas that were branded with this decal and the upgraded clutch were the L2950, L3450, L3650, L4350, L4850, L5450, M5700 and M6800.

Components of a Dry Clutch
Dry clutches make up the majority of those found in typical Kubota equipment, so it is these types that we will focus on for the remainder of the article. The dry clutch on your Kubota tractor comprises the following main components:

  • flywheel
  • clutch disc
  • pressure plate
  • clutch release or throw out bearing
  • clutch release fork

Flywheel
The flywheel is a heavy metal disc that is attached to the crankshaft at the rear of the engine. It is precision balanced and of a substantial thickness – typically around 1.5-2″ thick up to 3″ thick on a Kubota M series. These flywheels weigh about 40lbs in a small L series, 55-65lbs in a Grand L series and upwards of 125lbs for the larger 100HP M series.

Flywheels in your Kubota are 1.5-2" thick and typically weigh around 40-50lbs. They store momentum which steadies varying amounts of torque placed on the input shaft of the transmission by the engine.

Flywheels in your Kubota are 1.5-2 inches thick and typically weigh around 40-50lbs. They store momentum which steadies varying amounts of torque placed on the input shaft of the transmission by the engine.

The flywheel stores momentum which means it is able to steady the input shaft of the transmission by smoothing out fluctuating amounts of torque placed upon the shaft by the tractor’s engine. It also has a ring gear with teeth that accepts the starter motor’s drive gear when cranking the engine. A common Kubota service complaint regarding the flywheel is poor starter motor engagement from a worn motor pinion or a badly worn ring gear. In this case, often a shudder is felt when releasing the clutch on the tractor. This shudder indicates an imbalance in the flywheel due to general wear or a flywheel that has been machined down in the past and is now over its acceptable tolerance.

Point where the starter's drive gear (smaller gear in center) contacts the flywheel.

Point where the starter's drive gear (smaller gear in center) contacts the flywheel.

A flywheel that is not able to properly dissipate heat extremes will eventually stress-crack. The generated friction can also temper areas on the flywheel resulting in hot spots. These hot spot areas appear as blue burn marks on the surface of the flywheel and indicate a flywheel that is not handling heat properly.

A flywheel with stress cracks due to temperature extremes.

A flywheel with stress cracks due to temperature extremes.

A flywheel with friction hot spots burned into its surface.

A flywheel with friction hot spots burned into its surface.

Flywheel Resurfacing
If you are undertaking splitting of your Kubota tractor to service the clutch it is recommended that you resurface the flywheel – paying particular attention to the appropriate minimum thickness requirement. Kubota further states that the maximum amount of material that can be removed from any flywheel clutch surface is .060” (1.5mm). Post a comment if you require a specification for your Kubota’s flywheel and we’ll reply!

Clutch Disc
The clutch disc is comprised mainly of compressed asbestos with metal filings. This disc is held tightly against the flywheel by the pressure plate. The clutch disc is always in contact with the flywheel and as such, is subject to wear and slipping. Some common Kubota tractor service complaints with the clutch are slippage – due to disc wear, contamination from oil or from hot spotting on the flywheel.

Schematic of a Kubota clutch disc.

Schematic of a Kubota clutch disc.

Oil on the Disc
Oil can contaminate the disc from a bad rear engine main seal, or a bad transmission input shaft seal. Either way, oil on the disc is not good. Oil can also enter the clutch housing area and contaminate the clutch disc by over filling the transmission. The L-1 Series Kubotas, L2050, L2550, L2650 and so on, have a vent hole near top of the transmission, above the input shaft and contained in the clutch housing. Overfilling the transmission/common reservoir will push oil into this area which would then contaminate the clutch disc.

Riding the Clutch
Operators can prematurely wear out the clutch disc in their Kubota tractor by “riding” the clutch pedal. This results from not removing your foot from the pedal when shifting gears. You’ll know the smell of a burning clutch when you smell it – it smells expensive. Avoid riding the clutch by keep your foot on the foot board.

When to Replace the Clutch Disc
Signs that your clutch disc needs replacement:

  • inability to shift the transmission with the engine running without grinding the gears
  • the tractor will not pull a load
  • the tractor will not go up a simple incline
  • slipping, extreme shudder or squealing
  • expensive burnt clutch smell

Storing your Kubota? How to Prevent the Clutch from Rusting to the Flywheel
Kubota recommends that when storing your equipment, have the clutch disc pressure relieved by way of placing a block under the foot board after the clutch pedal has been depressed. This relieves the pressure on the clutch and prevents it from rusting to the flywheel. A clutch rusted to the flywheel is common service complaint is for owners of seasonal Kubotas (only use it in the winter to snowblow or only in the summer to mow). They may use their Kubota so infrequently that when they do attempt to start the tractor, the clutch and transmission are frozen – rusted together.

Kubota recognized this service issue and shipped all L175, L185, L235, L1500, L1501, L1801 and L2000 tractors with a small wooden block, painted black included in the under seat tool box. The block of wood had a yellow decal on it that indicated the correct placement of the block in keeping the clutch pedal depressed during storage. A paper tag with wire was attached to the clutch pedal further reinforcing its use if the tractor was stored and not used for any period of time. Later models like Daedong and Kubota’s joint venture, the 02 Series (L1802, L2002 etc), had a spring button that when pressed down, held the clutch pedal and kept the disc off the pressure plate.

Condensation & Free Play Considerations
Condensation in the clutch housing also creates a problem for the clutch disc. There is a drain at the bottom of the housing that should be opened from time to time to let water egress. If you have an early L175/L1500 and there is no drain plug, drill a small hole at the bottom of the housing to necessitate the removal of condensate from the clutch. Finally, clutch pedal “free play” should be adjusted from time to time as the tractor ages and the disc wears. Post a comment if you require a specification for your equipment.

Pressure Plate
The job of the pressure plate is to maintain constant, even pressure on the clutch disc, holding it in contact with the flywheel. The pressure plate has a precision-ground machined surface that the clutch disc contacts on the transmission side. At the centre of the pressure plate is an array of tensioned spring fingers. When compressed by the clutch release bearing, tension is alleviated on the disc plate and it is then free to rotate, independent of the flywheel. This allows the operator to change gears, or direction of travel.

Some common Kubota service complaints with the pressure plate are similar to those of the flywheel:

  • shudder – from hot spotting on the pressure plate’s machined surface
  • a mushy feeling – indicative of broken or worn springs
  • excessive pedal travel
  • inability to change gears
  • grinding of the transmission even though the clutch pedal is fully depressed
A schematic of a Kubota pressure plate.

A schematic of a Kubota pressure plate.

Clutch Release Bearing and Shift Fork
The clutch release bearing is a precision roller bearing that rides on the transmission input shaft of your Kubota tractor. It is held in place and activated by the clutch fork which in turn is attached to the clutch pedal linkage.

A close up shot of the clutch release bearing.

A close up shot of the clutch release bearing.

The clutch fork straddles the clutch release bearing holder.

The clutch fork straddles the clutch release bearing holder.

The large flat face of the release bearing presses against the spring fingers of the pressure plate to free the clutch disc. Some common Kubota Service complaints with the clutch release bearing are squealing, grinding and chatter when the clutch pedal is lightly pressed, or pressed a little too far (clutch pedal stop is not adjusted properly) and the squeal can persist during travel with the clutch fully engaged.

B Series Kubota tractors have a serviceable release bearing. A grease zerk is accessible from the side of the transmission tunnel. A few strokes keeps that bearing moving freely. Use care not to dislodge the small return spring attached to the bearing.

All or Nothing
If you find yourself in a situation that necessitates the splitting of the tractor to get at the clutch then you should consider replacing all of the serviceable components in there. Do not just install or have installed a new clutch disc. Replace it with a matched pressure plate and have the flywheel machined to the correct spec. Emery cloth and a lot of ambition does not machine a flywheel. When it comes to a Kubota clutch it is best to take an all or nothing approach to its repair – you’ll thank yourself in the long run.

That wraps up our overview of the different types of clutches in Kubota’s equipment and an in-depth look at the mechanics of the dry clutch found in most Kubotas. Read more about your clutch in the related articles below.

Service Department Vic

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Forum: Service, Repair & Maintenance

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