Drive Sprockets & Bell housings are covered on our introductory chainsaw maintenance courses, both for Domestic (one day) and for professional users (2 days) leading to either Lantra Awards ITA or City & Guilds NPTC Units 201 Chainsaw Maintenance.
The drive sprocket engages with the chain of the chainsaw to transmit the engine power and rotation via the clutch and bell housing. This rotates on the engine crankshaft which is supported by a needle or roller bearing.
The chain, guide bar and sprocket are all integral parts of the cutting attachment. They all wear out albeit at different rates. Take into consideration that cutting conditions and operator maintenance might make a difference. A handy rule of thumb is that a new sprocket might be required every 2-3 chains and a new guidebar every 4-5 chains.
Two sprocket types we typically find on the engines of the professional chainsaws we use on courses (Husqvarna 550XP and Stihl 261) are a rim and spline and spur or star. (as shown in the photos below.) These may be mounted either facing outwards or inwards depending on the manufacturer and model, and this is referred to concerning whether the centrifugal clutch can be seen when the side casing is removed (outboard) or is hidden (inboard). Generally, the Husqvarna saws have outboard clutches, and Stihls have an inboard, but there are exceptions – e.g. Stihl MS201TC has a lateral clutch, and Husqvarna 572XP has an inboard clutch – confusing!
Spur sprockets are integral with the bell housing, and on many Stihl entry-level models, these are fitted with the clutch inboard. Wear levels can readily be determined and if a replacement is required removal is straightforward – prize the E clip off the recess in the crankshaft, remove the washer, and the bell housing should pull off. Remember to release the chain brake, and on the machines lacking in maintenance, some force may be required to overcome the build-up of debris on the inside of the bell housing (clean this off when the component is off the machine). The bell housing can then be fully inspected. To refit, grease the roller bearing, engage the oil drive wand in the cut out of the bell housing (listen out for a definitive clunk), and then refit the washer and press the E clip back into place – we use pliers. Watch out for flying E clips...
Rim sprockets are separate from the bell housing and fit onto it via a splined profile. This means that when the sprocket is worn, only that component requires replacement. The bell housing can be re-used many times. Most professional Husqvarna models have these rim & spline arrangements with the clutch outboard. A little movement laterally on the splines also allows a smooth chain drive. Together with the engine speed the sprocket ratio also determines chain speed, this can be altered (just like changing the pedal set of gears on a push bike) if desired by fitting a sprocket with more teeth (faster, lower torque) or fewer teeth (slower, more torque). You need to understand how this might impact any PPE effectiveness as the minimum Class 1/20m/s leg protection may be below that required to stop a faster chain.
Removal of the sprocket with an external clutch is more involved as the clutch has to spin off (reverse thread) before the bell housing and sprocket can slide off the crankshaft. This is achieved by either hitting a screwdriver at the points indicated on the clutch (with decompression valve shut) or by using the manufacturer's tool to engage with the clutch and an impact drive. The piston may need stopping so that it stops rotating and a range of proprietary piston stops can be applied after removing the spark plug. Be careful not to damage the piston (holes are a possibility) or chop off a plastic piston stop in the exhaust port with this method.
Normal wear occurs due to the metal on metal contact and the inertia of the chain. The resistance of the chains movement on the material, accelerating and decelerating vs the drive of the engine. The chain drive links fit into the star shape or slots, and the chain tie straps sit on top of the star or rim surface and wear towards the centre. So, how much wear is acceptable? On an Oregon rim sprocket, handy narrow cut indicator lines across the sprocket will be visible until the sprocket is worn out. On a Stihl machine a gauge can be used to measure the depth of wear, and if the pins on the gauge fully fit into the wear groove, then it’s time to replace – about 0.5mm in both cases.
The bell housing is relatively robust, but check it for cracks after cleaning. It can be beneficial to place it on a screwdriver and tap it with another metal tool to listen for a clear ring – any muted or shortened sound might indicate a problem. Although in many years of chainsaw use, we’ve only seen a handful of bell housing failures. These failures had no warning and rendered the machine inoperable until fixed. The photo shows a Husqvarna 55o XP rim & splince sprocket snapped.
Significant heat can build up due to both the friction of the clutch acting on the inside of the bell housing and the chain brake operating on the outside. A lack of clear space to aid airflow can also contribute to heat build-up and in the photo below this has resulted in a marked distortion to the bell housing, as well as a tell-tale change in colour.
All this friction from the clutch and chain brake can lead to a thinning of the bell housing surface, so visually inspect and compare with a new one.
Key point – don’t run a new chain on an old sprocket or a new sprocket on an old chain, as it accelerates the wear on both!