Contenu

• Automatic Transmission Basics
• Controlling the Clutches
• Clutch Slippage
• Excessive Low RPM
• Excessive High RPM

There are a lot of myths out there on Planet Car, many of which are based on either outdated facts or assumptions built around incomplete data. The question of rpm-related transmission damage is an interesting one, since it does happen -- just not in the way you might think.

Automatic Transmission Basics

An automatic transmission consists of three basic sub-assemblies: the torque converter that transfers power from the engine to the transmission, the central shaft and clutch assembly that transfers power from the torque converter, and the planetary gearset at the rear, which receives that power. Of the three, the central shaft and clutch assembly is the most complex. The center shaft is actually a shaft within a shaft. By using clutches to lock either the inner or outer shaft to the case or to the engine, the transmission alters which gears in the planetary set remain stationary and which ones spin with the engine.

Controlling the Clutches

A regular automatic transmission uses a high-pressure hydraulic system to engage the clutches on the center shaft, and an electronic servo system to control the hydraulics. The clutches are actually a "clutch pack," consisting of several alternating friction discs (locked to the central shaft) and metal rings, locked to the clutch-packs outer drum. Hydraulic pressure pushes the clutch pack together, locking the shaft to the outer drum by sandwiching the metal plates between the discs. In this way, the transmission uses friction to transfer power to the road.

Clutch Slippage

Clutch slippage is death for any automatic. Clutches slip when theres too much torque going in, and too much resistance compared to movement. In practical terms, this can be seen when a very heavy load is being pulled by a powerful engine. Clutches tend to slip in higher gears and overdrive, when the input shaft has less mechanical advantage -- or leverage -- on the output shaft. This, among other reasons, is one reason that you should never tow a heavy load in overdrive, and why auto-equipped tow vehicles usually have a massive transmission oil cooler. Slight clutch slippage overheats and thins the oil, causing the clutches to lose grip and slip even more.

Excessive Low RPM

Contrary to popular belief, transmissions are more subject to damage by excessively low rpm than high rpm. Provided that the engine is running at high rpm because of either the rear-end gearing or because its in a lower gear, the mechanical advantage of that lower gearing will actually put less strain on the clutches. Drag racers often take advantage of this fact by using high rear-end gearing to compensate for a slightly weak transmission. So, in terms of clutch life, youre actually more likely to cause immediate damage to the transmission by running at lower rpm than higher rpm.

Excessive High RPM

While extremely low rpm and high loads will damage your transmission right away, sustained high rpm may damage it over the long run. High rpm means more wear on the bearings and oil seals, and quicker transmission fluid breakdown. At high rpm, the transmissions oil pump can overheat the fluid by constantly "shearing" it between the pumps gear teeth. The same thing can happen in the torque converter, though its less likely as long as the car is moving. So, the lesson here is that if you intend to run the transmission at consistently high rpm, then you should invest in a good oil cooler and an anti-wear additive for the transmission fluid.