What are the differences between the Alta and Helix 15% pulleys?

What is the difference between press-on and clamp-on?

 

the stock pulley is a press-on. It holds because the bore in the pulley is a few thousands " less than the shaft diameter. You could calculate the radial gripping force from the tensile strength and the amount of stretch to get it over the shaft. Typically, this is the strongest holding method. the pulley is insatlled by heating it up so the bore expands, gripping the shaft upon cooling. Extremely seure, but harder to install.

Browning-Morse, an industrial transmission drive specialist, developed a two part taper-fit hub and pulley system and that is what most of the Mini pulleys today have adapted, in several versions.

As far as the Alta, Helix and Promini, many, many hundreds of each are functioning flawlessly. They are both entirely made of stainless steel, unlike a few others using dis-similar metals.

The Vee groove design is CAD generated and CNC machined for optimum belt grip and has been from day one.

 

I didn't know that. Considering that, I would think that once the OEM pulley was taken off, a new press-on version would be near impossible to align correctly outside the factory.

On Randy Webb's site he says the following about the Alta: "First, it is all steel. What that means is that the thermal expansion rates are the same between the pulley, the hub of the pulley, and the shaft of the supercharger. That becomes very important when the temperature deltas can vary by as much as 300F. The same thermal expansion rate means you won't have to worry about the stability of the taper bore fitment on the shaft, or the tension between the hub and pulley. Secondly, it is stainless steel, so the unit will not corrode over time like aluminum and mild steel do."

Then on the Helix site it says: "The 2-part modular pulley includes a high-tensile carbon steel (70,000 ft/lb) hub which presses onto the supercharger shaft, just like the factory system. The aircraft aluminum pulley then bolts onto the hub, allowing the user to interchange pulley sizes, including returning the pulley size to stock."

Am I to assume that heat from the shaft is contained within the hub and not distributed to the aluminum-outer of the pulley thereby eliminating unwanted expansion? With the Helix, I like the idea of being able to change out pulley sizes and test different configurations, but the more parts, the more materials, the more percent error.

So, I would imagine that these two pulleys don't differ much in terms of performance (I would think the extra weight of the all steel Alta is negligible), but I would think there was a difference between the two in belt failure and/or pulley longevity. Is that a correct assumption?

 

a couple of corrections:

1. the shaft is steel, probably high strength.
2. the Alta and the current Helix pulleys are stainless steel and both are taper fit.
3. the original Helix pulley had a shrink fit steel hub (properies chosen to match the shaft, and a bolt-on aluminum wheel (heat expansion not critical) and was designed to fit properly when flushed to the shaft end, like the stock pulley.
4. the taper fit pulleys are easy to changeout for other sizes, as is the original press fit
(for a "300 degree delta", the blower shaft would have to be 350+ degrees... i doubt it, even for a few moments)

a few comparative bits:
aluminum: co of expansion 13 (in/in/def Fx 10exp-6)
stainless: 9 (in/in/def Fx 10exp-6)
bronze: 11 (in/in/def Fx 10exp-6)
steel: 6 (in/in/def Fx 10-6)
so proportionally, aluminim is about twice the expansion of steel and stainless is 50% more.

since the alta and Helix pulleys are stainless and the shaft is steel, if tightened at 70 deg F room temp, when they reach the 200 degree F operating temp, the stainless bore will increase (130 x 3 x 10exp-6) about 1/2 thousandth of an inch. the aluminum would have grown about .001"

 

Too bad the Helix doesn't have any good manufacturer support on this board


Actually they are both excellent pullies offering no problems for individuals.

 

the Helix pulley has six cap screws rather than just four so your getting more for your money:smile:

 

I spoke with a local installer that has done various different pulley's on MCS. So the installer was not the seller of the pulley. I inquired about pulley's and these were his comments so take it for what it is worth.

1) he did not feel that all pulleys were created equal.
2) he felt the RDR was one of the better ones because he felt the design gripped better the supercharged shaft and algned up easier.
3) he felt the 6 screw design was better than 4. He recently had swapped out a 4 screw because it had cracked.
4) he also commented that the grove design that grabs the belt were not created equal. He indicated that he has seen greater belt wear on other pulleys.

Having shared this (this is what lead me to buy the RDR), it is evident that all different brands are being used by NAM members with good results.

bahamabart

 

That's not a very significant difference in expansion. I'm going to go with the Helix pulley, so that I have more options down the road. That, and in speaking to Eric the other day, I really liked his no-unecessary-frills approach to product. If only I could afford his twincharge kit...

 

I would say that most likely the 4 screw one cracked (if it even did) because it was improperly installed or it was a crummy pulley. None of the reputable 4 or 6 screw designs have shown a flaw. Randy has installed well over a 1,000 of different pulleys and Helix has installed who knows how many, and none of them have had any failures.

 

Why a pulley would crack is beyond me!

I simply wanted to share what the installer shared with me and note that he did not sell pulleys merely installed them. I am not trying to "dis" anyones pulley. I think that if there was a "bad" pulley on the market, NAM memebers would have identified it by now.