Transmission Shafts & Gears

The internal combustion engines that propel most cars and trucks develop peak torque and usable power within a relatively narrow rpm range, and their rpm limit is generally about 6000 rpm. If engine power alone was relied on for propulsion, acceleration from a standstill might be difficult or slow at best, and top speed would not only be limited but be at an rpm that would be detrimental to acceptable fuel economy and engine longevity. However, if engine power is directed through a transmission, the gears in the transmission will enable the engine to stay within the rpm range where it makes the best power and operates most efficiently. The gears in a transmission multiply engine torque for brisk acceleration and allow the vehicle to cruise at highway speeds at an rpm that provides best fuel economy and engine durability. If you’re in need of transmission gears, shafts and related components, we have the parts to restore shifting performance and transmission reliability.

A transmission employs several combinations of drive and driven gears to multiply engine torque. The number of rotations that the drive gear makes in relation to the driven gear is the gear ratio. The greatest amount of torque multiplication is needed when the vehicle accelerates from a standstill and this is achieved with gear ratios like 3:1 or 4:1, which are typical for 1st gear in a transmission, where the engine power supplied by the drive gear is applied to a larger driven gear. The driven gear rotates 1 time for every 3 or 4 revolutions of the engine crankshaft and drive gear. However, with such ratios torque is gained at the expense of speed, and more gears with different ratios are needed.

Once the vehicle is underway, less torque multiplication but greater vehicle speed is needed, hence the transmission drive and driven gears become closer in size and rotational speed. Second gear may be 2:1 or 3:1, where the crankshaft and drive gear rotate 2 or 3 times for every one revolution of the driven gear, and 3rd gear 1:1, which is direct drive. In direct drive the drive and driven gears are the same size and rotate at the same speed. This was typically “high” gear in older 3-speed and 4-speed transmissions, but this ratio could still force the engine to turn at a high rpm at highway speed, especially if the differential gear ratio was numerically high, reducing engine life and fuel economy. For this reason, modern transmissions have overdrive gears, with typical ratios of 0.85:1 to 0.65:1. In overdrive the driven gear spins faster than the drive gear, therefore the engine can operate at a lower rpm while the vehicle is at highway cruising speed.

While the gear principles are the same for automatic and manual transmissions, the way that gear ratios and gear changes are achieved are vastly different. A typical manual transmission has an input shaft, an output shaft, and a countershaft that rotate on ball bearings and roller bearings inside a cast aluminum case. The input and output shafts are in-line with one another, with the countershaft below them. The gears on the input/output shaft and countershaft are in constant mesh and rotate with one another, but the gears on the output shaft can rotate freely on the shaft until locked in position. Power flow is from the input shaft, through the countershaft to the locked gear and the output shaft.

Between gears on the output shaft are hubs that are splined to the shaft. Over the hub is a slider, or sleeve, that is connected to the shift linkage. When the driver selects a gear with the gearshift lever, the shift fork moves the slider toward the gear, locking it to the hub and output shaft. Each gear has a coned surface that works with the cone on a synchronizer ring. When the slider forces the synchronizer against the gear, it quickly brings the gear to the speed of the shaft to prevent gear clash. A clutch assembly, consisting of a flywheel, clutch disc, pressure plate and release bearing, is positioned between the engine and manual transmission. The clutch allows the engine to be disconnected from the transmission so the vehicle can come to a stop while in gear with the engine running, and so the driver can make gear changes.

Instead of a clutch, automatic transmissions have a fluid coupling device called a torque converter that transmits engine power to the transmission. It serves the same purpose as the clutch, in that it allows the engine to continue running when the vehicle is brought a stop while in gear, except the torque converter does this without requiring any action on the part of the driver. Automatic transmissions use planetary gearsets consisting of a sun gear, planet gears encased in a planetary pinion carrier, and a ring gear or internal gear. The sun gear is so-named because it’s in the center of the gearset, where it meshes with the planetary gears that revolve around it. Three or four planet gears are in a carrier and the ring gear surrounds the carrier. The planet gears are in constant mesh with the sun and ring gears. Most automatic transmissions have at least 2 planetary gearsets, combined in a compound gearset, to create the necessary gear ratios. To achieve the desired gear ratio, one planetary member is driven by the engine while another is held stationary, with the remaining member becoming the output. Clutches and bands, applied by hydraulic pressure, are the components used to control the planetary members.

Clutch packs consist of multiple, alternating steel and friction material-lined plates housed in a drum or the transmission case. The friction plates are internally splined to fit into grooves on a hub, while the steel plates have external splines that engage grooves in the drum or case. A piston compresses the plates, locking the hub and drum together when hydraulic fluid pressure is applied. Return springs disengage the piston when pressure is removed. Bands are friction material-lined flexible metal rings that surround and are used to stop rotating clutch drums. One end of the band is attached to the transmission case, while the other is engaged by a hydraulic piston in a servo. The servo contains a spring to disengage the band when hydraulic pressure is removed. Hydraulic pressure in the transmission is developed by a pump driven by the torque converter and then routed to the valve body. Valves or electric solenoids in the valve body direct hydraulic pressure to the appropriate bands or clutch packs to shift gears.

Worn or defective gears can cause noise and poor shifting performance. On a manual gearbox, shift through all the gears to determine which gear is faulty. Noise from worn gears and bearings will get louder as speed is increased and bearing noise will sound like a growl. Jumping out of gear can be caused by shift linkage that is worn or out of adjustment, worn synchronizers, worn shift forks, or a transmission or bell housing that is misaligned. Hard shifting can be caused by a worn or misadjusted clutch, worn or misadjusted shift linkage, and binding synchronizers. On an automatic transmission, noise in 1st or 2nd gear that disappears when the transmission shifts to higher gears may indicate a defective planetary gearset. When the transmission is disassembled, all gears should be checked for worn, chipped and broken teeth. On a manual transmission, inspect the synchronizer teeth on the gear for evidence of rounding and the cone area for nicks, gouges and roughness that would inhibit smooth engagement. Check the gear and shaft journal areas for scoring or other damage. On automatic transmissions, check bushings and thrust areas for damage and check the planet gears for looseness or binding in the carrier.

No matter what you’re working on, we have the transmission shafts & gears needed to restore proper transmission operation and vehicle performance or upgrade it. Replacements are manufactured according to original equipment specifications, so when repairs are completed you can count on the performance your vehicle was designed to deliver. In addition to input shafts, turbine shafts, output shafts, main shafts, countershafts, reverse gear shafts, cluster gears, planetary gears, and reverse idler gears, we offer thrust washers, spacers, snap rings, synchro sleeves, synchro hubs, synchro rings, synchro keys, synchro springs, shift forks, drive chains, sprockets, carriers, carrier hubs, differential pinion gears, differential ring gears, differential sun gears, center supports, clutch hubs, clutch housings, and much more.