1 Brief Introduction to One-Way Clutches

1.1 Alternative Names for One-Way Clutches

One-way clutches for starters have many names in literature, such as one-way device, throw-out wheel, one-way clutch and drive gear, clutch assembly, one-way clutch and drive gear assembly, drive gear, transmission mechanism, one-way pulley clutch, one-way overrunning clutch, and pinion gear. The author believes that "one-way clutch and drive gear assembly" is a more appropriate term, but since most literature refers to it as "one-way clutch," this article also adopts this terminology.

1.2 Composition and Classification of One-Way Clutches

A one-way clutch consists of two parts: the drive gear and the clutch. Based on differences in internal structure, one-way clutches can be divided into 3 types: roller-type one-way clutches, spring-type one-way clutches, and friction-plate-type one-way clutches. Their appearance and function are identical. The drive gear and the clutch form an integrated component, which generally cannot be disassembled for repair.

1.3 Function of One-Way Clutches

During starting, the one-way clutch engages (meshes) with the engine's flywheel ring gear and transmits torque, driving the engine to rotate at high speed so that the engine starts and runs. At the end of the starting process, the one-way clutch disengages from the engine's flywheel ring gear to prevent the engine from driving the starter to rotate, thereby avoiding damage to the starter.

1.4 Operating Principle of One-Way Clutches

During starting, the energized electromagnetic switch generates magnetic force, causing the switch core to move. This actuates the shift fork to push the one-way clutch along the armature shaft. The gear of the one-way clutch engages (meshes) with the engine's flywheel ring gear, transmitting the torque generated by the motor to the engine. The engine rotates at high speed, allowing it to start and operate.

At the end of the starting process, the electromagnetic switch loses power and magnetic force. The shift fork returns to its original position. Combined with the centrifugal force of the one-way clutch itself, the one-way clutch also resets and disengages from the engine's flywheel ring gear, awaiting the next start.

1.5 2 Common Parameters of One-Way Clutches

There are 2 common parameters for one-way clutches: one is the number of teeth on the drive gear, and the other is the number of keys on the one-way clutch (i.e., the number of splines on the armature shaft). These are generally marked on the product packaging to facilitate selection, in the format: Number of Teeth × Number of Keys.

1.6 Precautions for Installing One-Way Clutches

When installing a one-way clutch, if it is too dirty, do not soak it in gasoline, diesel, or other liquids. Only locally clean the spline area or bushing area to avoid cleaning oil entering the interior and causing faults in the one-way clutch. During installation, apply a small amount of engine oil or a thin layer of grease to the spline area and bushing area of the one-way clutch. In winter in Northeast China, apply a small amount of anti-freeze engine oil or a thin layer of anti-freeze grease to prevent the grease from becoming too viscous due to low temperatures, which would make it difficult for the one-way clutch to slide.

2 Bushings for One-Way Clutches

The drive gear end of the one-way clutch is usually fitted with a bushing. The standard clearance between the armature shaft and the one-way clutch bushing is 0.03~0.09mm, and the maximum allowable clearance is 0.23 mm. The interference fit between the outer diameter of the bushing and the bushing hole is 0.08~0.18 mm. If the bushing wear exceeds the limit, it will cause unreliable engagement with the engine ring gear, noisy engagement, gear tooth breakage, and other faults. If wear exceeds the limit, it should be replaced. Table 1 lists the models of bushings for one-way clutches and their applicable vehicle models or general/applicable starter models. If the bushing cannot be purchased, it can be machined from a copper rod or iron rod.

3 Inspection, Repair, and Replacement of One-Way Clutches

1) The inspection of one-way clutches mainly involves checking torque transmission. Clamp the armature (rotor) in a vise, use a pipe wrench to hold the drive gear of the one-way clutch, and rotate it in the direction of torque transmission of the one-way clutch. If it slips with little force, the one-way clutch is unusable; if it does not slip, the one-way clutch is usable. This method provides only a rough estimate. For new parts and starters without slippage, inspection is generally unnecessary.

2) There are few parts of a one-way clutch that can be repaired. A common issue is slight damage to the end face of the drive gear, which can be welded and reused. If only burrs appear on the tooth surface, it can be repaired with an oil stone and reused. Internal faults of the clutch are generally not disassembled for repair, as they are difficult to fix after disassembly. Another common issue is excessive wear of the one-way clutch bushing, which simply requires replacement.

3) The dimensional requirements for each part of the one-way clutch are precise, making it difficult to find substitute models. Generally, only one-way clutches of the same model can be used for replacement.

For current reduction-gear starters, most one-way clutches are designed and manufactured by the starter manufacturers themselves and are proprietary products. When purchasing, careful comparison is required. It is advisable to bring calipers for careful measurement, and it is recommended to buy original factory parts.

4 Faults Caused by One-Way Clutches and Troubleshooting

Starter faults caused by the one-way clutch include: (1) A "rushing" sound is heard inside the starter when it rotates, but the engine is not driven to rotate. (2) The one-way clutch sometimes fails to engage with the engine flywheel ring gear and sometimes engages. Whenever it engages, the engine can be started, irregularly. (3) The vehicle can be started, but there is always a gear-clashing sound between the one-way clutch and the engine flywheel ring gear. (4) The electromagnetic switch operates, but the one-way clutch cannot engage with the engine flywheel ring gear. Table 2 lists the fault phenomena, causes, and troubleshooting methods for starter faults caused by the one-way clutch.

5 Quality identification of one-way clutches

One-way clutches of poor quality mostly have scattered splines in the clutch part and an excessively short service life. Their packaging is simple, mostly yellow cardboard boxes without printed text. The parts lack stamped brand and model markings, and the manufacturing is rough.

High-quality one-way clutches feature exquisite packaging with printed text on the cardboard box, including brand, model, and manufacturer-related information, as well as markings for the number of teeth and keys. The part is generally sealed in a plastic bag; upon opening, anti-rust oil is found. The part has stamped brand and model markings, fine manufacturing, and a long service life.

Comparing high-quality and low-quality one-way clutches side by side makes the differences obvious at a glance.

Published by Weidou Intelligent Technology (Shanghai) Co., Ltd.