"326" Water Pump Shaft Failure

[Joe C.]

Re: "326" Water Pump Shaft Failure

you can remove every other vane off of the impeller like i used to do to prevent cavitation at high RPM on race engines. back then there were no different size pulleys to reduce the pump speed

The pump doesn't cavitate. If it did, there'd be evidence of spalling on the impeller vanes. When a pump cavitates, areas of low pressure are created which causes the fluid to boil in localized areas. When the fluid "flashes" or boils, tiny pieces of metal are carried away from the vanes. Pumps which have been cavitating to any great extent look as if they are severely corroded, due to the spalling.

Vane type pumps, since they are not positive displacement, will "churn" the fluid if it is running at moderate to high speed and the flow rate through it is diminished. This can happen if there is a blockage in a closed system (like an engine's cooling system), or flow through a hose is shut off (such as on a fire pump). If the fluid is recirculated through the volute with low or no flow, it will become heated, sometimes to the point of inducing cavitation even though the impeller is not running at a high rate of speed. The best way to combat this situation, which can occur frequently in a fire pump, is to maintain flow by opening the pump drain partially................

[Joe C.]

Re: "326" Water Pump Shaft Failure

Joe,

Taking a close look at that shaft, it looks odd in that the outside of the break is smooth, and the inside is crystallized. Looks like something put a knife edge on the outside, wore the shaft diameter down to a point where it couldn't take the load and broke.

Maybe a foreign object or some part of the bearing assembly.

Was there any evidence of a foreign object or parts broken off the bearing housing?

Looks to me like the outer annulus went through repeated cycles of bending stress until it failed in tensile, and then the inner section later failed in shear due to torsion.

An expert metallurgist or someone well versed in failure analysis should easily identify the mechanism(s) that were at work here.

There was no evidence of a foreign object either within the bearing or within the vane/volute.

[Clem Z.]

Re: "326" Water Pump Shaft Failure

The pump doesn't cavitate. If it did, there'd be evidence of spalling on the impeller vanes. When a pump cavitates, areas of low pressure are created which causes the fluid to boil in localized areas. When the fluid "flashes" or boils, tiny pieces of metal are carried away from the vanes. Pumps which have been cavitating to any great extent look as if they are severely corroded, due to the spalling.

Vane type pumps, since they are not positive displacement, will "churn" the fluid if it is running at moderate to high speed and the flow rate through it is diminished. This can happen if there is a blockage in a closed system (like an engine's cooling system), or flow through a hose is shut off (such as on a fire pump). If the fluid is recirculated through the volute with low or no flow, it will become heated, sometimes to the point of inducing cavitation even though the impeller is not running at a high rate of speed. The best way to combat this situation, which can occur frequently in a fire pump, is to maintain flow by opening the pump drain partially................

how we found the problem in the BBC race engines. we installed a pressure gauge in the intake manifold water passage next to the thermo housing and we were only seeing about 10 PSI at 6500 RPMs but after modifying the impellers we saw over 40 PSI. we ran a 5/8" diameter restrictor instead of a thermostat. the lower pressure was not filling the head water cavities completely and we saw overheating and detonation from hot spots in the combustion chambers.

[Clem Z.]

Re: "326" Water Pump Shaft Failure

this message showed up in my mail box but not on this site.
---Quote (Originally by Clem Zahrobsky (45134))---how we found the problem in the BBC race engines. we installed a pressure gauge in the intake manifold water passage next to the thermo housing and we were only seeing about 10 PSI at 6500 RPMs but after modifying the impellers we saw over 40 PSI. we ran a 5/8" diameter restrictor instead of a thermostat. the lower pressure was not filling the head water cavities completely and we saw overheating and detonation from hot spots in the combustion chambers.---End Quote---Did the problem occur before or after the pump impellers were modified? before as this is why we did the mod. i was working with the engineers at GM as this was a early 396 BBC we were using in a dirt late model race car.