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Tracor Pulling Clutches Header

Tractor Pulling Clutches

This article presupposes your Puller is fitted with a Steel Flywheel. If not then please read the Steel Flywheels article first!

Unless you happen to be very handy with a Torque Converter* then you will end up using one of these in your Puller. Further down it says clutches come in two flavours but before you say anything, I have deliberately ignored all the oddities like Cone clutches, Dog (or pawl) clutches, Wet clutches, etc. because they are not used in the sport of Tractor Pulling.

Clutches come in two flavours: -

Both have their good and bad points which I will summarise at the end of the article. Now back to basics; how does a clutch work?

Tractor clutch componentsTractor clutch operation

Conventional Clutch

The default state of this clutch is engaged (on). The first illustration shows the components of a standard single-plate friction tractor clutch and the second shows you how the system actually works. By default the clutch is engaged, in other words you have to push down the clutch pedal to disengage it which is the norm.

In terms of Tractor Pulling a single friction plate is fairly rare. In the majority of cases dual-plates are used to increase grip and spread and dissipate the heat over a wider area.

Sorry, this section is still under construction...

Centrifugal Clutch

Centrifugal chainsaw clutchThe default state of this clutch is disengaged (off). Apart from mopeds and chainsaws, this type of clutch is also used in other applications where the speed of the engine defines the state of the clutch, for example Tractor Pulling. This clutch system uses centrifugal force to automatically engage the clutch when the engine rpm rises above a threshold and automatically disengages when the engine rpm falls low enough.

The moped and chainsaw version looks similar to a drum brake. The friction pads (or shoes) are radially mounted and engage the inside of the rim of a housing (drum). A spring or springs hold the shoes away from the rim until the centrifugal force caused by the engine rotation stretches the springs, forcing the mechanism to engage.

That little digression aside, back to Tractor Pulling. Centrifugal clutches come in two principal varieties, fully Automatic (e.g. CrowerGlide) and Pedal Clutches (Crower Pullmaster).

Crower 'CrowerGlide' Clutch - exploded view

 

How does a centrifugal Pulling clutch work?

How the arms work in the clutch

Sorry, this section is still under construction...

Clutch friction plates

This is the point at which things do start becoming a bit more technical. Sorry about that but it sort of comes with the territory!

The clutch disc ensures a smooth transition during every engagement, while also standing up to the abuse of this action. If the clutch disc is going to last, it must be covered with a friction material that: -

  1. Can withstand the torque and RPM of the engine.
  2. Can withstand the clamping force of the pressure plate.
  3. Can manage the heat caused by engagement and disengagement.
  4. Finally, the clutch is usually required to engage and disengage smoothly (racing clutches, not so much).

Part and parcel of these requirements are the two friction coefficients which relate to points 1 and 4 above.

In the good (or should that be bad) in view of Asbestosis old days asbestos was frequently used a the disk friction facings, due to its ability to withstand high temperatures. Modern clutches typically use: -

Organic

These use a compound organic resin with copper wire facing (for heat dispersion). The compound is a mix of fibreglass and other materials moulded or woven into a friction pad.

Organic facings are typically made from phenolic resins, friction modifiers like metallic powder or metal oxides, and compounded rubber. These facings come in two types: -

Moulded Facings, which are very affordable but lack strength. When tested at 500° F, an 11" OD/6.5" ID moulded facing will burst at 5,000 RPM. This type is totally unsuitable for Tractor Pulling.

Woven Facings, which include fibreglass yarn woven into the material to increase strength. The burst strength of an 11" OD/6.5" ID woven facing is over 10,000 RPM.

Ceramic

Ceramic materials are typically used in heavy applications such as racing or heavy-duty hauling, though the harder ceramic materials increase flywheel and pressure plate wear. Ceramic clutch material, which is mostly a mix of silicon dioxide and various metals and additives, sintered or brazed onto the clutch disc.

Ceramic clutch facings can withstand considerable heat - they can operate without fading at temperatures up to 538°C (1,000°F). This heat resistance makes them ideal for racing. Finally, it's important to note that the ratio of static to dynamic friction is quite high for ceramic clutches. This means that ceramic clutch engagement can be abrupt, which in theory makes them totally unsuitable for Tractor Pulling.

Kevlar

Kevlar (and it's cousin Twaron), which are synthetic fibers that make for extremely long-lasting (and very forgiving) clutch friction pads. Kevlar is the registered trademark for a para-aramid synthetic fibre ([-CO-C6H4-CO-NH-C6H4-NH-]n), related to other aramids such as Nomex and Technora. Developed by Stephanie Kwolek at DuPont in 1965, this high-strength material was first commercially used in the early 1970s as a replacement for steel in racing tires.

Kevlar® and Twaron® facings last 2-3 times longer than organic facings, all things being equal. Additionally, these fibers have a low static-to-dynamic friction coefficient, making them an ideal choice for applications where smooth engagement is essential (such as Tractor Pulling, off-road driving, rock crawling, etc.).

Feramic clutch material

Feramic clutch material, which is fairly similar to ceramic material, except containing a much larger percentage of metal. The facings are made from a combination of steel, silicon dioxide, tin bronze, and graphite. Feramic facings can be full across the face or they can be buttons. Feramic clutch material is great for motorsport, as it has high coefficients of static and dynamic friction. It "hooks up" very fast, which is important in racing, but it's terrible on a daily driver, as it makes smooth shifting impossible, especially at low speeds.

FeramAlloy

FeramAlloy, which is a new and superior alternative to feramic and ceramic clutch material. FeramAlloy will probably replace ceramic and feramic clutch materials, as it offers the same key benefits (high static friction coefficient, high fade temp), but with a much lower dynamic friction coefficient that makes changing gear much smoother.

In order to understand the differences between clutches in terms of engagement and durability, Phoenix Friction have assembled some friction coefficient (expressed as "µ") and fade temp data below. Basically, the higher the temperature, the more severe the application.

Phoenix Friction Materials Data Table

This Friction Material Data Table has been 'borrowed' from one of the Phoenix Friction Products pages and this is well worth a visit! Click this link to visit this Phoenix Friction page.

Pro's and con's

Conventional Clutch

Centrifugal Clutch

Under construction... Under construction...
   
   
   

UK Clutch Inspectors

If you need to have your clutch inspected and are a member of a Pulling Association or Club this should be your first port of call...

All of these are ETPC approved and are members of the BTPA, which is the ruling body for Competition Tractor Pulling in the UK.

Aberdeenshire David Gall david@drgtechnology.co.uk
Buckinghamshire Peter Clarke declarke3@btconnect.com
Cumbria
David Todd doggtodd@gmail.com
Lancashire John Eccles simplyredpuller@btinternet.com

Centrifugal Clutch Manufacturers

Primary sources