Tapered vs. Straight Shaft Pulley I/F

[Darth]

Hi all

Need help to determine if adapting a pulley from a tapered shaft to straight bore will work. Can the clamping force be calculated for a tapered shaft to bore i/f and straight shaft? I have a professionally modified Porsche crankshaft in which 10mm of the tapered end was cut off creating a straight or snub nose crankshaft allowing earlier version pulleys to be mounted to the end. Previous owner modification allowing the newer 3.6L engine to fit in an earlier model Porsche. I'd like to retain the original 3 sheave pulley because it is also a harmonic damper but is over 50% heavier (i.e. 6.4lbs) than the heaviest pulley (i.e. 4lbs) ever mounted to a straight shaft of previous years. The centre section of the pulley is conveniently held in by 8 bolts making it very easy to replace it with a re-machined straight bore adapter centre section. My concern is whether the lock created by the face of straight shaft is enough to support the heavier pulley which originally had a greater lock created by the interference fit of the tapered shaft? The modified tapered shaft retains the original larger 14mm internal bolt bore threads vs. 12mm allowing greater torque and in turn higher clamping forces. The modified shaft face also has a larger 5mm vs 4mm index pin off centre to the bolt bore. Up until recently I thought the shear strength of this pin was significant but was told otherwise by 3rd party pulley manufacturers that the surface i/f created the majority of the lock. If more details are needed please don't hesitate to ask.

TIA

 

[JRMichler]

We need diagrams of the original and modified crankshaft and pulleys. And what is i/f?

 

[Darth]

i/f = interface
image 1 - tapered bore pulley illustrating removable center section - 6.4lbs
image 2 - Straight shaft end
image 3 - Straight shaft end dimensions
image 4 - Tapered shaft – unpolished end is approximately 10mm and has been machined off and a new 5mm index pin hole drilled off center to crankshaft bolt bore to accept early style pulley, tapered to 27mm
image 5 - straight bore pulley - 4lbs
Straight shaft bolt - M12x1.5x22 torqued to 170Nm = 125 ft lbs
Tapered shaft bolt - M14x1.5x40 torqued to 235Nm = 173 ft lbs

 

[Ranger Mike]

i would not hesitate to use the 14mm bolt. If you are not revving the engine past 6500 rpm should be ok. The small block chevy V8 race engine uses 7/16 bolt. This is straight shaft 8 pound damper bolt. I would safety wire it .second choice is lock tight thread locker. Personally i would put another shear pin in as well.

 

[Darth]

Unless I miss a down shift the 6500 rev limiter is still in place. The original 14mm bolt threads were deep enough down in the shaft that they were not compromised when the 10mm's of taper was removed. Think of the shaft as starting out as a straight or snub nose shaft (i.e. Porsche had the same shaft for years only the internals changed like stroke etc.) and Porsche decided to add 10mm's of taper to the end and at the same time increase the bolt size from 12 to 14mm and in turn increase tightening torque. They also went to a smaller 4mm index pin which contrary to my beliefs provides minimal shear strength (i.e according to after market pulley manufacturers trying to sell light aluminium racing pulleys and is primarily there for aligning timing marks) compared to the lock created by the torque on the bolt and the friction at mating surfaces. I was told that the snap back on a throttle blip is what may cause the pulley to back off and loosen, but again contrary to my beliefs I would think it would be the torque required to drive the fan, alternator and a/c compressor would be more signficant. The subsequent model was essentially the same engine (i.e 3.6L) but they beefed up the original lightened tapered shaft to get rid of the 4th order harmonics, did away with the 6.3lb harmonic damper / anchor and replaced it with a solid 2.5lb straight bore pulley. The length remained 10mm longer. Maybe the extra 10mm length is to provide more lateral support? All the earlier shafts and this one protrude from the engine case a few mm's and it's just the connection at the face of the crankshaft (i.e 29.965mm diameter) that creates the lock. I trust Porsche must have had a good reason to go to a longer tapered shaft but I don't know why so I came here to get help to crunch some numbers.

 

[Dr.D]

What you may do to the torsional systems response is very hard to predict. The really regrettable part about that is that, even if you have made a bad mistake, it will not be evident until enough revolutions have been acquired for torsional fatigue to give you a failure. I strongly suggest staying with the factory design unless you are able to do a full forced torsional fatigue analysis.

 

[Baluncore]

There is no question that the pin or alignment dowel is only there to maintain timing mark alignment. The tension in the axial bolt holds the taper tight and that taper locks the pulley to the shaft. Any movement at the taper I/F is bad and will shear the alignment dowel. Use as much of the existing taper as possible. Make sure the taper is clean before assembly, maybe use a locking compound.

Google “Taper Lock” pulleys. They are used in industry to lock pulleys tight onto parallel shafts.

 

[Darth]

I strongly suggest staying with the factory design unless you are able to do a full forced torsional fatigue analysis.

That's too easy. I was thinking of removing the alignment pin for a non-destructive test to determine the amount of torque required to cause the pulley to slip in each case.I have both crankshafts and pulleys and if I torque each bolt to spec and somehow put a torque wrench on each pulley, I can measure the torque required to break the lock.

There is no question that the pin or alignment dowel is only there to maintain timing mark alignment.

A service machine shops that specialize in these cars offer, is double pinning...so it must add some lock and I would think the further away from the centre the greater the shear strength.

Use as much of the existing taper as possible.

All of the taper has been removed except for about 1mm which appears to be a chamfer

 

[Baluncore]

I have both crankshafts and pulleys and if I torque each bolt to spec and somehow put a torque wrench on each pulley, I can measure the torque required to break the lock.

You will not be able to do that by hand with a torque wrench. The taper will lock tight so long as there is tension in the axial bolt, any movement will tighten the taper. Knowing the taper you could calculate the break force.

A service machine shops that specialize in these cars offer, is double pinning...so it must add some lock and I would think the further away from the centre the greater the shear strength.

The machine shop will do what they get paid to do. If you do not have a taper lock then you must have a keyway to carry the load. Pins will not do the job unless there are 6 or eight thick dowels.

All of the taper has been removed except for about 1mm which appears to be a chamfer

Removing the taper will have weakened the shaft by forming a sudden step change in diameter. The taper did a much better job of locking the pulley and transferring energy along the shaft.

 

[Rx7man]

I don't see the step there creating a weakness that is going to affect anything, it's not on the drive side of the engine where it's critical.. The lack of a taper however is going to be problematic..

I see two options.. get a full keyway cut into it and a matching pulley, or build up the missing taper with weld and have it ground back down to size, allowing you to use it again.. or both.

 

[Baluncore]

I don't see the step there creating a weakness that is going to affect anything, it's not on the drive side of the engine where it's critical.

The crankshaft was designed to do only what was needed with the minimum weight. Reducing the weight must encroach on the design specifications, QC and testing. You cannot know the number of revolutions before fatigue failure will occur. Dismissing the concern because it is not part of the driveline is irrational. The motor will overheat without ancillary drive to the water pump.

The front pulley carries the sideways belt tension and the harmonic balancer energy exchange. When the front of the shaft fails, it will be at a step in diameter, at the RPM with most energy coupling to the harmonic balancer.

Welding should not be used to build up a crankshaft as it generates heat affected zones that initiate cracking. Metal spraying onto the rotating shaft can be used because it produces a more homogeneous material. Unless you have the equipment, it would probably be easier, cheaper and more reliable to replace the crankshaft.

You might be able to make an adapter with an internal cylindrical surface to fit the shaft, and an external taper to fit the pulley. That adapter would need four or more thin slots to transfer external taper force to the cylindrical shaft, like the inner part of a Taper Lock pulley. An adapter would not strengthen the stepped shaft.

 

[Darth]

Knowing the taper you could calculate the break force.

What more do I need to know about the taper to calculate the break force?

 

[Baluncore]

What more do I need to know about the taper to calculate the break force?

Firstly; Given the pitch, diameter and torque of the draw bolt you can calculate the bolt tension.

Secondly; The taper rate and the friction coefficient of the surfaces. The taper surface mean diameter is important, but the surface area is not needed for the computation.

If the tangent of the included taper angle is greater than the friction coefficient, a static taper will release when draw bolt tension removed. Lesser angle tapers self lock. But in a dynamic situation with rotation and side forces, draw bolt tension is essential to keep the taper locked in place.

Where you have steel on steel you can significantly increase the friction coefficient by inserting a thin paper sheet between the surfaces. Cigarette papers are accurate and available. There are also locking compounds formulated for tapers.