Intermediate Shaft Straight-Cut Gears

[atelier14]

Hello,

I am considering a set of JBRs 'Intermediate Shaft Straight-Cut Steel Gears' for my 2.8L MFI engine that is currently build.

Is the noise difference really that extreme compared to the factory gears? Would you use these for a car that sees 70% street and 30% DE use?

Thanks,
Christian

[AlfonsoR]

Ahhh. This is a question I have floating around in my tiny brain as well, so I'm also interested in the answer from the experts.

Just curious, why are you considering straight cut? For your information (in case you did not know), the 964 angle cut gear is made of steel, i believe.

[chris_seven]

This is an interesting question and I have looked at making straight cut intermediate gears for some time but always backed away because I was not certain I could sell them.

The theory is that straight cut (spur) gears absorb less power than helical gears but when there is only one gear pair I think it would be difficult to measure.

Single helical gears do cause thrust particulary when there is any torsional vibration present as this will reverse the dirction of thrust and cause serious problems. Double helical gears have no thrust and are quiet.

This was a particular issue with high speed single cylinder engine dynos at around 20 000rpm.

The 1/2 order torsional vibrations produced by the engine used to damage the helical gearboxes that connected the engine to the dyno. Swaping to spur gears solved the problem as they do not transmit thrust.

The noise issue associated with spur gears is complex. Gear noise is always proportional to the pitch line velocity of the meshing gears so the faster the speed the worse the noise.

As gears come into mesh there is an impact force (sometimes known as a landing force) and if there is a pitch error in the gear cutting then noise can be quite high. In fact this problem is magnified at low loads. This noise can be reduced by grinding a relief onto the tooth tip but this operation does add cost.

The tooth form and the ability of the gear to slide rather than stick-slip is also important and accurate profiling is essential as is surface finish hence grinding or shaving.

The same issues will also affect a helical gear but the landing forces are much less of an issue. With a spur gear the entire tooth makes contact at the same instant (providing the gear has been cut square). A helical gear has a much longer meshing period and engages gradually which much reduces the landing force. There are also more teeth in engagement at any one time so pitch inaccuracies tend to be less important.

It is difficult to predict the absolute noise level of a spur gear as the noise is really a function of how well it has been made and this is more critical with straight cut proflies.


We have two manufacturers of Ford Competition gear sets here in Engalnd and they make straight cut gear kits used in a siginificant number of Race and Rally cars. You can always tell one manufacturers gears from the other due to a huge difference in noise.

[Flieger]

Would crowned spur gears give no thrust and an intermediate noise?

[Tippy]

The noise would be welcome in my car. If anyone wants to give me one for experiment, I'm game....

[mamut]

I have used straight cut gears from JBR, there is a tiny bit of noise from them, never have bother me whatsoever,cheers.

[BURN-BROS]

I've run straight cut gears and have not heard any overwhelming noise. Transmission gears however....

[cgarr]

Originally Posted by BURN-BROS

Transmission gears however....

I was in the paddock when an old Mini came off the track, the tranny sounded like my lathe in back gear!

[chris_seven]

Originally Posted by Flieger

Would crowned spur gears give no thrust and an intermediate noise?

Max,

I don't think spur gears produce any thrust but I am sure there are techniques for reducing thrust.

I had always thought to make these gears with an involute form and make them a spur design.

It seems from most of the comments that there isn't much of a noise issue so it would be a good profile to use.

I think I will re-price them and make a few sets.

[strokher racing]

MIke at JB is the one to ask on the straight cut gears. He does not try to upsale anything when he is building a motor for you. Stand up guy and shop!

[gestalt1]

So, is there a noticeable hp gain for strait cut gears?

[chris_seven]

A good estimate for the power loss of a spur gear would be about 1% of the transmitted power and about 1.25% of a helical gear.

To measure the difference I think you would need a very accurate valve train test rig as a full engine dyno probably has too much uncertainty.

A typical valve train rig would be driven by a electric 30 hp motor and even if all this power was transmitted the spur gear would only absorb about 0.3hp and the helical gear about 0.375hp.

I think it is more to do with reliability at high rpm than power losses.

[Walt Fricke]

I went to straight cut gears mainly because both were steel. Having the teeth strip off of an aluminum gear is an experience one does not want to repeat. Then, of course, there was the lure of a bit less parasitic loss. Though Chris is suggesting this is negligible. And I didn't know I could just use the steel 964 gears.

In a race motor noise is not a concern - at least if you don't run noise limited events with very low limits.

[chris_seven]

Walt, I agree about the 964 gears but the current price from my OPC means that I can still have them made as spurs for a lower price and I still like the idea of a reduction in axial thrust on a race motor.

[Walt Fricke]

Chris

Reduced thrust (on both IS and crank) was also part of my thinking, too. The thrust surfaces on the bearings for both of those have always shown more wear than the main bearing surfaces. On the crank the clutch can be a contributor, of course.

I can't say that I have noticed a measureable reduction in bearing thrust surface wear after moving to the straight cut gears, but that's due to low sample numbers (like one) as well as the fact that bearings tend to get replaced when the case is opened up, need it or not.

[lucittm]

Porsche engineers are a strange lot. They will select the best design despite the cost. Then the marketing gurus will change the design to lower the cost. Sometimes the engineers will go back and fight for their design. In this case, I don't know if they went back to fight for the helical-cut gears but that is what we got. I think the VW ended up with straight-cut gears. In any case it is far more expensive to design and manufacture helical-cut gears, so they are that way for a reason and the reason is not cost.

Mark

[chris_seven]

Originally Posted by lucittm

In any case it is far more expensive to design and manufacture helical-cut gears, so they are that way for a reason and the reason is not cost. Mark

I can' see why designing a helical gear is any more expensive than a spur gear and once you have a gear cutting machine that will produce helicals the cost difference is negligible. It is, of course easier to find sub-contratcors to cut small quantities of spur gears and I think this is the main reason most after market egars use this design.

The standard gear is, of course, aluminium and as such I can believe that a helical would be chosen for conservative reasons. Helical are stronger for a given thickness and have lower surface loads so why take the risk.

I also agree with Walt that as the basic loading is small the thrust will be small but if noise isn't the issue the steel gear is much stronger than the aluminium gear why not use a spur and get rid of the thrust. particulalry on a high rpm race engine.

[YTNUKLR]

How many people have munched the aluminum gear? I'd venture to guess, on the street, extremely few, and on the race track, still not many. The stock aluminum helical is quiet and it is light and it is still pretty durable.

It seems the aluminum gear gets damaged in sustained high rpm use. This makes sense. Friction (from sustained high rpm)generates heat and the aluminum fatigues and eventually fails.

Porsche DID use the straight-cut steel gears in the 73-75RSR. What do they use now, in the GT3RSR ? Lightweight versions of the 964 Steel-Helical. They must think the reduction in noise or vibration gained from these constant-mesh gears is worth it. They do not use the straight-cut gears. Maybe because Porsche Motorsports tears them down all the time anyway.

Interesting to note though, because the helical cut gears and the small bearings for the intermediate shaft seem to be the cause of the IS bearings grinding to copper much sooner than main bearings. I see the main problem with the helical IS gear though, is not the thrust of the shaft (that's what she said), but the torque perpendicular to the axis of the IS, that wears the bearings out and generates more friction.

I think the steel spurs are worth more than 0.25% of the power of the engine compared to a helical gear; seems like there is a lot more friction on the IS bearings due to the helical design, than there is friction loss due to the teeth design itself...

[Walt Fricke]

Lunching an aluminum IS gear once was enough for me. 12 broken rockers instantly. Don't recall how many valves needed replacing - I must have repressed that part of things, one way or the other.

[chris_seven]

Originally Posted by YTNUKLR

How many people have munched the aluminum gear? I'd venture to guess, on the street, extremely few, and on the race track, still not many. The stock aluminum helical is quiet and it is light and it is still pretty durable.

I agree that for street applications the helical gear must be a good idea as it does seem relaible and works well.

Originally Posted by YTNUKLR

It seems the aluminum gear gets damaged in sustained high rpm use. This makes sense. Friction (from sustained high rpm)generates heat and the aluminum fatigues and eventually fails.

I think high speed creates surface pitting which eventually leads to a fatigue failure. I don't think temerature will vary much beyond the bulk oil temperature.

Originally Posted by YTNUKLR

Porsche DID use the straight-cut steel gears in the 73-75RSR. What do they use now, in the GT3RSR ? Lightweight versions of the 964 Steel-Helical. They must think the reduction in noise or vibration gained from these constant-mesh gears is worth it. They do not use the straight-cut gears. Maybe because Porsche Motorsports tears them down all the time anyway.

I am not sure noise is much of an issue but vibration must be a concern and this could be modified by using the optimum pressure angle.

If the gear isn't heavily loaded and bending fatigue isn't a major issue then the noise/vibtration generated by a spur gear can be reduced by using a shallower pressure angle. At 15 degrees the gear could run quite smoothly.

Originally Posted by YTNUKLR

Interesting to note though, because the helical cut gears and the small bearings for the intermediate shaft seem to be the cause of the IS bearings grinding to copper much sooner than main bearings. I see the main problem with the helical IS gear though, is not the thrust of the shaft (that's what she said), but the torque perpendicular to the axis of the IS, that wears the bearings out and generates more friction

Looking at the basic angles involved the radial force on the bearings would increase by about 6% for the helical gear compared to a spur gear.

Interstingly the axial force calculates to about 6% as well so it works out about 50/50 thust and radial.

Originally Posted by YTNUKLR

I think the steel spurs are worth more than 0.25% of the power of the engine compared to a helical gear; seems like there is a lot more friction on the IS bearings due to the helical design, than there is friction loss due to the teeth design itself...

0.25% of the engine power equates to around 3/4 of a horsepower which seems high but I can agree this needs a bit more wok to try to define.