Stretched Chain Scrapes Sprockets | Check Sprockets When Chain Deflection Increases!

Even with regular lubrication, wear is inevitable for drive chains that are pulled with great force during acceleration and deceleration and bent by the front and rear sprockets. As the chain stretches, deflection increases, but at the same time, wear on the sprockets progresses. When adjusting the tension of a sagging chain, it is important to check the condition of the sprocket teeth as well.

The wear of the pins and bushings causes the chain to stretch and the rollers to pitch wider.

To check the amount of chain deflection, swing the chain up and down in the middle of the front and rear sprockets. The deflection of the Yamaha Tricker (2004 model) is 40 to 45 mm, and is measured at the point of greatest tension, taking into account the unbalanced elongation of the chain.

As the chain stretches, the number of deflections increases, so a chain puller is used to adjust it to the proper value. It may seem as if the problem has been solved by keeping the deflection within the specified range, but the pitch will not mesh well with the sprocket due to the widening of the pitch caused by pin wear. If the chain lifts off the tooth flanks when picked up on the driven sprocket, it can be determined that the chain is stretched.

When the chain stretches due to wear of the pin and bushing, the spreading is not the same for the outer link with the pin press-fitted and the inner link with the bushing press-fitted. The pitch of the inner link, whose pitch is fixed by the inner plate, is difficult to change, while the pitch of the outer link, which connects adjacent inner links, tends to widen due to pin wear. The contact surface between the chain and sprocket, where the pitch changes unevenly, is random due to the different number of teeth in the front and rear, and the sides of the tooth flanks are gradually scraped off.

As you know, cleaning and lubrication on a regular basis are important to reduce drive chain elongation. Lubrication of the contact area between the bushing and the pin is important for chains with small-sized plates connected by pins, and sealed chains have been put to practical use to ensure this lubrication.

In addition to the pins and bushings, lubricating components are also needed between the rollers, which contact the sprocket tooth surface, and the bushings located inside the rollers, and chain manufacturers who perform various severe tests specify lubrication every few hundred kilometers.

However, even regular lubrication will not eliminate wear on the pins and bushings. This is true even for seal chains that are filled with grease for lubrication. Although the surfaces of the bushings and pins are hardened by quenching treatment to increase their surface hardness and wear resistance, the strong frictional force exerted by the strong pull during acceleration and deceleration gradually wears the surfaces of both bushings and pins.

Chain elongation due to pin and bushing wear is indicated by an increase in deflection between the front and rear sprockets. Even if the chain adjuster is used to adjust the deflection to the correct amount, it is inevitable that the deflection will increase as the mileage increases. The increase in deflection is perceived as chain elongation, but a more concrete explanation of the cause of the elongation is the increase in deflection due to the widening of the roller pitch.

Pitch refers to the distance between pins and is denoted by the hundredths place of a three-digit number, such as 428 or 530, which indicates the size of the chain. Specifically, a 428 4 is four-eighths of an inch, and a 530 5 is five-eighths of an inch, which is the distance between the pins. Considering the amount of deflection due to pin wear, if 100 pins of 100 links wear by 0.1 mm, the total amount of deflection will increase by 10 mm. This elongation leads to an increase in deflection.

Wear of sprockets progresses as rollers with widened pitch bite into them.

Wear on the tooth flanks of the sprockets is more pronounced on the surfaces where the rollers hit hard. When traveling, the sprocket in the image rotates to the left, indicating that more of the anodized aluminum on the left side of the valley has been scraped off.

This is an example of a roller that no longer fits in the bottom of the sprocket and has cut the tooth surface at an angle due to running without adjusting the deflection even though the chain has grown longer and the pitch has widened. If further wear progresses at this rate, the tooth surface on the left side toward you will become even smoother and deformed like a nato, and the chain will not be able to hang on.

The wear on the drive sprocket tooth surface that pulls the chain is the opposite of the driven sprocket, which is pulled by the chain, and the chain rollers try to bite into the right side of the tooth surface.

The 2004 Yamaha Tricker has a drive chain size of 428 with 124 links. The wear check is determined by measuring the inner width of the bushing for 10 links. Since partial impacts from sudden acceleration or deceleration can cause uneven elongation, the chain should be measured at several points to determine when it is time to replace it.

The length limit in the service manual is 119.5 mm, while the actual measurement is also 119.5 mm, so it must be replaced. Even if a non-seal chain is lubricated at the manufacturer's recommended timing, a sealed chain is two to three times more durable than a non-seal chain. Durability here refers to the amount of time it takes for the chain to reach a specified elongation rate.

Pitch widening = pin and bushing wear is a typical pattern of chain deterioration, but the effect also extends to the sprockets. This is because the shape of the sprocket teeth is designed to match the pitch of a new chain.

When the chain and sprockets are new, the rollers of the chain move along the tooth flanks of the sprocket valleys in a tight fit. However, as the pins and bushings wear and the pitch widens, the width of the rollers becomes wider than the width of the sprocket valleys. The roller will no longer bite into the bottom of the valley and will transmit power while slightly floating above the bottom of the valley.

As the pins wear further and the pitch widens, the rollers hit the sides of the sprockets harder instead of the bottom of the valley, and the tooth flanks begin to thin. The chain then deflects more as it gets closer to the bottom of the tooth flanks.

Thus, the pitch expansion due to pin wear and sprocket wear progress synergistically, and the increase in deflection continues unabated. Some models have replacement limits marked on the chain puller at the rear end of the swing arm, but in most cases, both the chain and sprocket have reached the point of replacement when the puller is fully pulled out.

If only the chain is replaced at this time, the tooth flanks and pitch of the worn sprocket do not match and play in the direction of rotation that occurs while the chain is engaged, causing uneven wear on the rollers of the new chain that has been installed. The service manual for certain models specifies the replacement of the chain, drive sprocket, and driven sprocket as a three-piece set when the elongation of the drive chain exceeds the specified value.

It is a mistake to think that lubrication is futile, since lubrication of rollers and bushings and tooth surfaces of rollers and sprockets is effective in reducing their respective wear, while non-seal chains, as well as sealed chains with grease, fill the voids between pins and bushings, will still cause progressive wear on pins and bushings.

If the chain rotates smoothly and meshes smoothly with the sprockets, it is certain to last longer than if it continues to be ridden without care. Continue to perform proper maintenance by checking for increased chain deflection and wear on the sprocket tooth flanks.