"326" Water Pump Shaft Failure

[Gene M.]

Re: "326" Water Pump Shaft Failure

Joe,
It was good to meet you at national. Always better to have a face to connect with a name.

Just a thought, as I recall a needle bearing is designed for a Rc 58 hardness on the shaft surface ( I'm going back to my design years so may be off). As the metal is made harder it becomes more brittle. So could it be possible the stress risers if not drawn back cause issues as you experienced? I don't think the intent was to thru harden the shaft only a surface hardening was necessary for the needle bearing. Something about carbonizing to complete the hardening process sticks in my mind.

[Clem Z.]

Re: "326" Water Pump Shaft Failure

I respectfully disagree with you about your theory of unsupported weight. If the issue were pure cantilever stress from unsupported weight, then the fan/pulley/clutch assembly would have to weigh on the order of hundreds of pounds in order to bend or break a 5/8" diameter piece of hardened round stock. A very unlikely instance is where larger inertial mass would strain the shaft only when a very heavy assembly is either started or stopped abruptly. This requires very high acceleration values, and does not happen if the clutch is operating properly.

The problem(s) arise when the rotating mass is spun. Any unevenness in the distribution of that weight will cause stresses on the shaft along both the thrust and the radial axes. The forces/stresses produced will increase (probably as the square) of the imbalance point's radial velocity. When the material's modulus of elasticity is exceeded, then those stresses will remain as strains (deformations). You can see that the bending moment on the shaft at any time is proportional to (probably) the square of fan speed, weight value of the imbalance, and radial distance of that imbalance outward from the fan's center .

I don't have any of my college textbooks handy, so I take liberty in parenthesizing the word "probably" in a couple places, but you get the idea. If a practicing engineer with a better memory than mine cares to do the mathematical analysis, then that would be a very interesting exercise.

my idea of weight if spinning and is slightly off center would cause stress,bending motion on that shaft. if your clutch was a rebuilt was it rebalanced ?

[Timothy B.]

Re: "326" Water Pump Shaft Failure

Joe,

The next time your feel racey take the fan off.

[Joe C.]

Re: "326" Water Pump Shaft Failure

Joe,

The next time your feel racey take the fan off.

Not allowed in the class I am in. Must be stock or appear to be stock.

[Joe C.]

Re: "326" Water Pump Shaft Failure

my idea of weight if spinning and is slightly off center would cause stress,bending motion on that shaft. if your clutch was a rebuilt was it rebalanced ?

Right, but only to the extent that that weight is unbalanced. Again, the only way that a well balanced, heavy rotating mass would need a stouter shaft would be if that mass were subjected to large acceleration during its duty cycle.

Does F. O. in FL balance them as part of the restoration process?

The fan blades would have a far greater sensitivity to a balance issue than the clutch, because the fan's inertial weight lies further out radially from the shaft's C/L.

[Clem Z.]

Re: "326" Water Pump Shaft Failure

Right, but only to the extent that that weight is unbalanced. Again, the only way that a well balanced, heavy rotating mass would need a stouter shaft would be if that mass were subjected to large acceleration during its duty cycle.

Does F. O. in FL balance them as part of the restoration process?

The fan blades would have a far greater sensitivity to a balance issue than the clutch, because the fan's inertial weight lies further out radially from the shaft's C/L.

i would have the fan rebalanced and maybe buy a old clutch that is shot from the stand point of coupling up and use that while racing. maybe find a way to disable the clutch maybe like remove the viscus fluid

[Joe C.]

Re: "326" Water Pump Shaft Failure

Joe,
It was good to meet you at national. Always better to have a face to connect with a name.

Just a thought, as I recall a needle bearing is designed for a Rc 58 hardness on the shaft surface ( I'm going back to my design years so may be off). As the metal is made harder it becomes more brittle. So could it be possible the stress risers if not drawn back cause issues as you experienced? I don't think the intent was to thru harden the shaft only a surface hardening was necessary for the needle bearing. Something about carbonizing to complete the hardening process sticks in my mind.

I dunno, but would guess that the shaft only needs to be case hardened to a few thousandth's, perhaps 0.010" maximum. Also dunno if it is tempered after hardening to increase toughness.

It would be great if a metallurgist could examine that break and come up with the answer. I still believe that I probably overtightened the belts, OR the bearing happened to be defective, or a combination of the two. Not sure of the Rockwell hardness necessary here.

I bought a product called Kasenit, and used it to case harden my distributor's flyweights. I have been unable to locate a set of original GM issue, which were case hardened, so I hardened the Accel weights. The compound works very well, and increases the carbon content of low carbon steel. Oil or water quench also changes the granular structure near the surface. Can't drill through the weights anymore using a standard high speed twist drill bit.

[Joe C.]

MORON the "326" Water Pump Shaft Failure

Moron the pump shaft failure.
Here is a similar pump shaft failure copied/pasted from another website:



Philip
Moderator
Join Date: Feb 1999
Location: Marana, AZ
Posts: 8,773


Re: Water Pump Shaft Failure
My youngest son had this happen 2 days ago. Shaft snapped in
the same place. It was a rebuilt pump on the engine less than a month. Took out the fan shroud, radiator, fan clutch and fan. This was on a 300k mile TBI 350 in an 89 K5 Blazer. I do not think it is capable of high rpms and has the serpentine belt system.

He is going to file a claim but in reality the manufacture will blame another component for the failure. I'll post photos when I get home.

************
Here are the photos:
Looks a lot like yours, Joe. Failure right at the bearing.







GM fan clutch and fan, rebuilt pump from O'Reillys about a month old installed in 300k mile 1989 K5 Blazer, belt was replaced same time the pump was. Nothing race about any part of that vehicle, the engine has never been apart, but is showing some signs of wear.

[Michael W.]

Re: "326" Water Pump Shaft Failure

It would be great if a metallurgist could examine that break and come up with the answer. I still believe that I probably overtightened the belts, OR the bearing happened to be defective, or a combination of the two. Not sure of the Rockwell hardness necessary here.

Joe-

If you look at your photo way up above showing the shaft remnant and pump innards, there is a spot on the shaft at about the 7 o'clock position. Is that an indent from impact, dust or maybe some spalling? Got a better pic?

[Gerard F.]

Re: "326" Water Pump Shaft Failure

"Expodential"!

It must be that hard water I used to cut the Prestone with.

"Under-driving" the pump would work. But "how do it know"?

Joe,

The pump shaft broke between the pulley and the pump impeller, so I'm not so sure the fan or fan clutch has anything, or little, to do with the shaft failure.

On any centrifugal pump, the main stress on the shaft is in torque created by force against the impeller. The force is due to friction through the cooling channels in the block. The faster you turn the pump, the higher volume being pumped and the higher the friction force. And the friction won't be completely linear at higher volumes or rpm.

Like I said, you redlined your pump, which was marginal to begin with at high RPM.

What could solve things is a larger bypass, slip clutch in the impeller
or a larger pump pulley to slow the pump rpm.

One could probably calculate the torque stress but you would need to be able to compute or measure the force on the impeller. A good mechanical engineering project for you.

Wadda you from, South Brooklyn

[Joe C.]

Re: "326" Water Pump Shaft Failure

Joe-

If you look at your photo way up above showing the shaft remnant and pump innards, there is a spot on the shaft at about the 7 o'clock position. Is that an indent from impact, dust or maybe some spalling? Got a better pic?

Good observation Mike............looks like spalling or galling, but could also be impact damage judging by the Chevy Orange in the vicinity. IT ALSO LOOKS LIKE IT COULD BE AN OLD FRACTURE, WHICH HAS BEGUN TO SHOW SIGNS OF CORROSION DUE TO COOLANT INTRUSION!

If it is spalling or galling, then I'll have a chance to re-examine the ball bearings and races when Howard sends everything back to me. I asked for the old parts back for further examination.

Spalling points to excess preload on the bearing which points to overtighened belts, imbalance, foreign object or lack of lube.

As far as pictures, these are the best I can do, for now:

[Joe C.]

Re: "326" Water Pump Shaft Failure

Joe,

The pump shaft broke between the pulley and the pump impeller, so I'm not so sure the fan or fan clutch has anything, or little, to do with the shaft failure.

On any centrifugal pump, the main stress on the shaft is in torque created by force against the impeller. The force is due to friction through the cooling channels in the block. The faster you turn the pump, the higher volume being pumped and the higher the friction force. And the friction won't be completely linear at higher volumes or rpm.

Like I said, you redlined your pump, which was marginal to begin with at high RPM.

What could solve things is a larger bypass, slip clutch in the impeller
or a larger pump pulley to slow the pump rpm.

One could probably calculate the torque stress but you would need to be able to compute or measure the force on the impeller. A good mechanical engineering project for you.

Wadda you from, South Brooklyn

Thanks Jerry.
The shaft broke right at the bearing
Those suggestions can be looked into.

[Clem Z.]

Re: "326" Water Pump Shaft Failure

Joe,

The pump shaft broke between the pulley and the pump impeller, so I'm not so sure the fan or fan clutch has anything, or little, to do with the shaft failure.

On any centrifugal pump, the main stress on the shaft is in torque created by force against the impeller. The force is due to friction through the cooling channels in the block. The faster you turn the pump, the higher volume being pumped and the higher the friction force. And the friction won't be completely linear at higher volumes or rpm.

Like I said, you redlined your pump, which was marginal to begin with at high RPM.

What could solve things is a larger bypass, slip clutch in the impeller
or a larger pump pulley to slow the pump rpm.

One could probably calculate the torque stress but you would need to be able to compute or measure the force on the impeller. A good mechanical engineering project for you.

Wadda you from, South Brooklyn

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

[Gerard F.]

Re: "326" Water Pump Shaft Failure

Good observation Mike............looks like spalling or galling, but could also be impact damage judging by the Chevy Orange in the vicinity. IT ALSO LOOKS LIKE IT COULD BE AN OLD FRACTURE, WHICH HAS BEGUN TO SHOW SIGNS OF CORROSION DUE TO COOLANT INTRUSION!

If it is spalling or galling, then I'll have a chance to re-examine the ball bearings and races when Howard sends everything back to me. I asked for the old parts back for further examination.

Spalling points to excess preload on the bearing which points to overtighened belts, imbalance, foreign object or lack of lube.

As far as pictures, these are the best I can do, for now:

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.

Looks a little like the interior of my wheel bearing failure and broken spindle:




The spindle shaft on the outside looks different, the inner cylinder bearings and housing broke loose, and actually rounded the outside of the spindle shaft before it broke:



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

[Clem Z.]

Re: "326" Water Pump Shaft Failure

https://www.forums.ncrs.org/attachme...64906&uid=8736 the smooth area is where it was cracked before it separated in the center