What causes a water pump’s shaft seal to fail?

It’s often the smaller components that can cause the most damage when things go wrong and a water pump’s mechanical shaft seal fits that bill. febi pinpoints what can go wrong and why.

With higher power requirements and reduced emissions on the agenda, newer commercial vehicle engines have to work more efficiently. The materials used are pushed to their limits, with higher pressures and temperatures. As a result, engines need to be cooled more efficiently.

In the cooling system, the water pump carries out the majority of this task. The pump’s successful operation depends on the smallest, most important and sensitive component, next to the bearing assembly that is. Yes, we’re talking about the mechanical shaft seal which can’t be seen from the outside but which deserves closer observation given its intricate make-up. The mechanical shaft seal sits on the water pump shaft and serves as a seal between water pump bearing and coolant. It is made up of many parts, some of which rotate and some of which are stationary (Fig. 1).

The sealing surfaces of both rings (rotating seal ring and stationary seal ring) slide against each other under the pressure of a spring so that sealing is guaranteed. The seal ring pairings are made from highgrade, wear-resistant materials, such as silicon, graphite or ceramic. The mechanical shaft seal’s service life is significantly shorter if savings are made on these materials. This is also the reason why, although similar on the outside, water pumps can be very different in quality.

Film stars

Constant lubrication and cooling between the two rings is necessary to guarantee that the mechanical shaft seal will function steadily throughout its service life. This is achieved via lubricating film which forms between the two pairs of seal rings during rotation. This 1-2 μm thick lubricating film should always be present both to reduce friction and also so that any heat produced can be dissipated (Fig. 2).

It is normal for a certain amount of leakage between the rotating seal ring and the stationary seal ring. However, this is very small and may even evaporate in the pump.

Sometimes coolant can also get into the free space behind the mechanical shaft seal and may escape at the drainage hole. This is normal in small quantities, particularly after initial fitting, and does not constitute any reason for complaint. Some pumps have evaporative canisters, where the escaping coolant is collected and kept in the pump until the liquid has evaporated. Continuously escaping coolant, however, is a sign of a faulty mechanical shaft seal and therefore signifies a leaky water pump (Fig. 3).

The mechanical shaft seal is a very sensitive part that can be damaged for the following reasons: 

1- Transport damage: the mechanical shaft seal may be damaged during transport or due to external impacts on the water pump shaft (e.g. knocks). 

2- The use of old and/or dirty coolant: in this case, rust, limescale or dirt may destroy the surfaces of the mechanical shaft seal. As a result, the sealing of the water pump bearing is no longer guaranteed. 

3- Dry running of the pump due to lack of coolant: the coolant lacks lubricating properties if the wrong mixing ratio of antifreeze to water is used. The result is that rotating seal ring and stationary seal ring dry rub against each other without the protective lubricating film. This causes friction heat which can destroy the mechanical shaft seal. 

4- Fitting errors: the use of additional sealant when fitting (silicone) can destroy the surfaces of the rotating and stationary seal rings. 

5- Dry running of the pump due to lack of pressure in the cooling system: the pressure in the cooling system may be too low if the valve of the radiator filler cap is faulty. The result is that the compression spring exerts too much pressure which means that both rings lie completely on top of each other. This prevents the formation of any lubricating film, and the mechanical shaft seal runs dry.