Why do some Crank and Cam Pulleys/Sprockets use a taper fit?

[HonorAndHonesty]

So the Ford Taurus SHO with the Yamaha engine used a taped fit on the cam sprockets and eventually they would come loose and trash the engine. The Hellcats have a taper fit on the crank pulleys and they are known to spin.

My question is why do some manufacturers use a taper fit and not have a traditional keyed sprocket or pulley? Is it weight, balance or something else?

 

[Skippy722]

Probably just cost savings/ease of manufacturing. Don’t have to mill a keyway into anything now.

 

[vavavroom]

So the Ford Taurus SHO with the Yamaha engine used a taped fit on the cam sprockets and eventually they would come loose and trash the engine. The Hellcats have a taper fit on the crank pulleys and they are known to spin.

My question is why do some manufacturers use a taper fit and not have a traditional keyed sprocket or pulley? Is it weight, balance or something else?

A tapered fit allows for more precise timing adjustment compared to a keyed fit. It's a feature, not a cost-saving measure.

 

[Skippy722]

A tapered fit allows for more precise timing adjustment compared to a keyed fit. It's a feature, not a cost-saving measure.

It’s relatively simple to cut keyways to the same accuracy as you’d get from a tapered fit.

 

[billt460]

The Chrysler Eco Diesel used a pressed on cam sprocket. It was / is notorious for coming loose and destroying engines.

 

[billt460]

It’s relatively simple to cut keyways to the same accuracy as you’d get from a tapered fit.

Especially with today's accurate, modern, CNC machining centers.

 

[vavavroom]

Consider even belt tension.

 

[billt460]

Consider even belt tension.

Both belt and chain driven cams have tensioners. What's to consider?

 

[vavavroom]

Both belt and chain driven cams have tensioners. What's to consider?

With the tapered fit design, you tension the belt before you tighten the sprockets down. The cams stay in alignment. If you have keyed cams the belt locks the cams in position but as soon as the tensioner applies tension the cams will go slightly out of alignment relative to each other.

 

[billt460]

With the tapered fit design, you tension the belt before you tighten the sprockets down. The cams stay in alignment. If you have keyed cams the belt locks the cams in position but as soon as the tensioner applies tension the cams will go slightly out of alignment relative to each other.

Camshaft misalignment isn't the issue with these non keyed, pressed on sprockets. Them coming loose and causing interference in the process is. A keyed shaft / sprocket prevents this.

They have been successfully used for years on all types of engines. Going to a friction, press fit was simply a cost cutting measure that obviously didn't work out.

 

[vavavroom]

Tapered fit is nothing new in engine design or in engineering. I would not expect machining precisely matching tapered surfaces to be less costly than maching precisely fitting splines or keyways. The problem is the interference engine design. Tapered fit cam sprockets are only additional but not very likely failure points in a flawed design. Belt/chain tensioners, bet/chain guides, failing, incorrect assembly during timing belt/chain replacement or timing belts breaking represent a larger and more likely number of failure points compared to keyless cam sprockets. I don't know about the particular Chrysler engine you mentioned but isn't the cam slippage related to the high pressure fuel pump that's cam-driven?

 

[billt460]

I don't know about the particular Chrysler engine you mentioned but isn't the cam slippage related to the high pressure fuel pump that's cam-driven?

As far as I've heard it's pertaining to the sprocket that is press fit on to the camshaft. It has a tendency to slip, and when it does it throws the valvetrain out of time. It's an interference engine, so you know the rest. $$$$$$$.

 

[dumkid]

press fit pulleys are better than the ford duratech/masda mzr crank pulleys that are held in time by a few crush washers