Ask Ray! Where to put the thinner steel plates in the clutch drum?

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February 4, 2019

Question: When installing steel plates of a different thickness in a clutch drum, does it matter where I put the thinner steel plates in the clutch drum?

Answer: It depends which clutch drum you are referring to.

Steel plates are often used to adjust clutch pack clearance or increase clutch pack space to add additional friction and steel plates for more capacity. Depending on the application, it can make a difference. First know whether the clutch drum works as a holding clutch or a shifting clutch.

A holding or static clutch, such as a reverse or forward clutch, applies and transfers torque usually while the vehicle is at rest. So under normal driving conditions they are typically not engaging or disengaging. The number of times a holding clutch applies is significantly lower than if it were to be used in a shifting application. A holding clutch needs to hold and transfer torque. When applied, very little heat is generated within the clutch pack; therefore thinner steel plates can be used anywhere in the clutch pack. Typical holding clutch failures result from insufficient torque capacity, hydraulic pressure leaks or pressure control issues. Forward to reverse abuse also contributes to failure.

An example of a shifting or dynamic clutch is an intermediate clutch or a 3-4 clutch. A dynamic clutch needs to transfer torque by coupling an inner hub to an outer hub, each rotating at different rpm's, during a gear ratio change or shift. During a shift, shifting clutches transfer larger amounts of energy than a static clutch. Therefore, a greater amount of heat is geneiate&during clutch apply, which produces higher temperatures at the friction and steel surfaces. The longer the slip, the higher the temperature climbs, making the engagement time a critical factor. In addition, the typical clutch pack with double sided frictions creates heat input on both sides of a steel plate during an engagement. The steel plate needs to absorb and dissipate the heat without warping, hot spotting or coning. The ability to do so is related to its thickness or mass.

During the shift, the temperature rise is generally higher in the center of the clutch pack than at either end, which makes it better to place thinner steels away from the center. Placing them against the apply piston or backing plate where temperature is limited is the best advise. That's why specialty clutch packs that include all thin steel plates, so you can add more clutches, actually overheat and fail sooner than stock plates. There's a lot of engineering needed to balance performance AND durability.

The next time you need to know, just "Ask Ray."

posted by: Nathan Truncone   comments: 0

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