24 replies to this topic

[Nial81]

I'm planning to rebuild a gearbox for my 1293 turbo. I was thinking of the mini spares heavy duty helical gearset however I'm concerned about the ratios and particularly having a tall 1st gear. At the moment it's a standard box with a X pin diff and 2.95 FD does anyone have experience of this and how do you find it.
Cheers

[Steve220]

I have the gearset on both my turbo minis, both running 3.1 FD on ATB diffs. The gear set is excellent! I've never had any issues running a tall first gear! 2.95 FD i'd say is probably not worth it. Either a 3.1 or 3.2 makes a much greater driving car.

 

[Nial81]

Thanks, yeah I plan to swap to a 3.1 when I build it. I wasn't sure how annoying it would be in traffic,it's already quite a heavy clutch with the rts. I'll probably go for it then , I definitely don't want to be going down the straight cut gears route.

[Java_Green]

I have also considered using the mini spares heavy duty helical gearset. However, my main concern is about the gearbox housing when putting a lot of torque through the gearbox. Straight cut gears do not produce force transfer losses ending up as axial loads and in forth gear it should be fairly quiet as the load goes straight through the mainshaft. Worst case, using helical gears, should be in second gear during "drive" (opposite to "coast") where the layshaft is pushed towards the centre web of the gearbox housing. (At this very moment I do not have a gearbox at hand to measure the different gear radius to calculate the actual axial force level based on typical torque figures) I guess second gear is a gear that is most likely to be used when using full engine torque...

 

Just me thinking....

[DeadSquare]

I have also considered using the mini spares heavy duty helical gearset. However, my main concern is about the gearbox housing when putting a lot of torque through the gearbox. Straight cut gears do not produce force transfer losses ending up as axial loads and in forth gear it should be fairly quiet as the load goes straight through the mainshaft. Worst case, using helical gears, should be in second gear during "drive" (opposite to "coast") where the layshaft is pushed towards the centre web of the gearbox housing. (At this very moment I do not have a gearbox at hand to measure the different gear radius to calculate the actual axial force level based on typical torque figures) I guess second gear is a gear that is most likely to be used when using full engine torque...

 

Just me thinking....

As you say, you are only just, thinking.  Continue thinking, more deeply. 

 

Look at the photo of the gearbox above and imagine the gears meshing under load.  As the power is transferred to the lay shaft, the helical cut of the gears exerts a lateral force that "pushes" the lay shaft towards the transfer web at the end of the gearbox.

[Java_Green]

 

I have also considered using the mini spares heavy duty helical gearset. However, my main concern is about the gearbox housing when putting a lot of torque through the gearbox. Straight cut gears do not produce force transfer losses ending up as axial loads and in forth gear it should be fairly quiet as the load goes straight through the mainshaft. Worst case, using helical gears, should be in second gear during "drive" (opposite to "coast") where the layshaft is pushed towards the centre web of the gearbox housing. (At this very moment I do not have a gearbox at hand to measure the different gear radius to calculate the actual axial force level based on typical torque figures) I guess second gear is a gear that is most likely to be used when using full engine torque...

 

Just me thinking....

As you say, you are only just, thinking.  Continue thinking, more deeply. 

 

Look at the photo of the gearbox above and imagine the gears meshing under load.  As the power is transferred to the lay shaft, the helical cut of the gears exerts a lateral force that "pushes" the lay shaft towards the transfer web at the end of the gearbox.

 

I followed your advise and thought it through ones more. You are right about that in 1st-3rd and R, the load path is from the 1st motion shaft to the laygear and from the laygear to the mainshaft. Using helical cut gears and during "drive" (2nd and 3rd gear), in the first engaged gearmesh the laygear is "pushed" towards the transfer web at the end of the gearbox (engine backside). However, in the second engaged gearmesh the laygear is "pushed" in the opposite direction. Since the second engaged laygear have a smaller diameter than the one that is engaged with the 1st motion shaft, and force ( as well as torque) equilibrium over the layshaft exists, force magnitude is larger in the second gear interaction than is the first. As the helical angle is the same for all gears on layshaft, the axial force component acting on layshaft is larger in the second interaction than in the first (with 1st motion shaft). Resulting axial force ends up to be pushing the layshaft towards the centre web.

 

Please correct me if I am wrong.....

[DeadSquare]

I would not presume to correct you, but permit me please, to add further food for thought.

 

If you were to very smoothly slice and lubricate an imaginary 3" bar at a "helical angle", and then pinch the ends between thumb and forefinger, the "helical" edges would be tempted to dissipate  end thrust into a lateral thrust.

 

If you were to now place the two pieces of bar on a smooth floor with one piece against a wall, and push on the end of the other piece with a broom handle, would the length of the broom handle make any difference to the end thrust and the subsequent lateral thrust?

 

I ask this because I am unsure, why you are preoccupied with the diameter of the gears.

[Midas Mk1]

What ratio is first gear? if it's anything like the minispares straight cut gearset i'd have a think of where and how you want to drive it.

I found hillstarts , especially those in traffic on a 3.1 and sccr 'fun' for about 6/7 years, finally bit the bullet last year and changed the fd, one of those where i'd wish i'd done it sooner.

[Java_Green]

I would not presume to correct you, but permit me please, to add further food for thought.

If you were to very smoothly slice and lubricate an imaginary 3" bar at a "helical angle", and then pinch the ends between thumb and forefinger, the "helical" edges would be tempted to dissipate end thrust into a lateral thrust.

If you were to now place the two pieces of bar on a smooth floor with one piece against a wall, and push on the end of the other piece with a broom handle, would the length of the broom handle make any difference to the end thrust and the subsequent lateral thrust?

I ask this because I am unsure, why you are preoccupied with the diameter of the gears.

Obviously, there is some misunderstanding. I am not sure I follow your example amd what it has to do with the subject. Initially I felt - why bother.... But, if somebody else actually is interested and want to know, the time is not wasted. I give it a try.

Let us stick to gearboxes. Their purpose is to up/down shift torque and rotational speed. Every shaft is (in every moment) in torque and force equilibrium. Let us focus on torque. Torque=Force*Level arm. On a geared shaft the level arm is the distance from centre of shaft to contact line (along gear flank), i.e. roughly the gear radius. The force acting on the gear flank (F) is in the gear flank surface's normal direction. For helix cut gears, this force can be divided in two components one in the tangential direction (Ft) and one in the axial direction (Fa).
Back to torque and equilibrium, 1st motion shaft is a simple example.
With 1:1 ratio over the transfer gears engine torque (Teng) is what reach the 1st motion shaft. In the other end there is a reaction force acting on the gear flank (in surface normal direction). So, the tangential component (Ft) times its level arm in radial direction (L, approximately = r) equals engine torque (Teng). {Teng = Ft*L => Ft = Teng/L}
For helix cut gears; (helix angle=alpha):
Fa = Ft*sin(alpha) = (Teng/L)*sin(alpha)
(For straight cut gears; alpha = 0 => Fa = 0)
This is the axial force acting in axial direction on 1st motion shaft towards the main shaft. At the same time it is the axial force (from this gearmesh) acting on the layshaft towards the end of the gearbox. When it comes to the layshaft, it also comes an axial force contribution from the second engaged gear pair. Continue setting up a torque equilibrium equation for the layshaft with the engaged gear pairs (2nd or 3rd) and you will figure out the resulting axial force and in what direction it goes.

[Steve220]

I have also considered using the mini spares heavy duty helical gearset. However, my main concern is about the gearbox housing when putting a lot of torque through the gearbox. Straight cut gears do not produce force transfer losses ending up as axial loads and in forth gear it should be fairly quiet as the load goes straight through the mainshaft. Worst case, using helical gears, should be in second gear during "drive" (opposite to "coast") where the layshaft is pushed towards the centre web of the gearbox housing. (At this very moment I do not have a gearbox at hand to measure the different gear radius to calculate the actual axial force level based on typical torque figures) I guess second gear is a gear that is most likely to be used when using full engine torque...

 

Just me thinking....

Not sure for others, but I rarely use full throttle in second. Mainly 3rd and 4th. I love the maths to this, a few people at work have crunched the numbers and it isn't as bad as i first thought. My other consideration of straight cut was the numerous gearboxes that have had the centre web split due to the gears trying to climb over each other at high torque loads. The evo helical sets are in a lot of high powered turbo A series engine set ups now. I know of at least 2 now running over 250lbft for a few years now. Time will tell on the cases, and bearings, as to the longevity!

[DeadSquare]

 

I would not presume to correct you, but permit me please, to add further food for thought.

If you were to very smoothly slice and lubricate an imaginary 3" bar at a "helical angle", and then pinch the ends between thumb and forefinger, the "helical" edges would be tempted to dissipate end thrust into a lateral thrust.

If you were to now place the two pieces of bar on a smooth floor with one piece against a wall, and push on the end of the other piece with a broom handle, would the length of the broom handle make any difference to the end thrust and the subsequent lateral thrust?

I ask this because I am unsure, why you are preoccupied with the diameter of the gears.

Obviously, there is some misunderstanding. I am not sure I follow your example amd what it has to do with the subject. Initially I felt - why bother.... But, if somebody else actually is interested and want to know, the time is not wasted. I give it a try.

Let us stick to gearboxes. Their purpose is to up/down shift torque and rotational speed. Every shaft is (in every moment) in torque and force equilibrium. Let us focus on torque. Torque=Force*Level arm. On a geared shaft the level arm is the distance from centre of shaft to contact line (along gear flank), i.e. roughly the gear radius. The force acting on the gear flank (F) is in the gear flank surface's normal direction. For helix cut gears, this force can be divided in two components one in the tangential direction (Ft) and one in the axial direction (Fa).
Back to torque and equilibrium, 1st motion shaft is a simple example.
With 1:1 ratio over the transfer gears engine torque (Teng) is what reach the 1st motion shaft. In the other end there is a reaction force acting on the gear flank (in surface normal direction). So, the tangential component (Ft) times its level arm in radial direction (L, approximately = r) equals engine torque (Teng). {Teng = Ft*L => Ft = Teng/L}
For helix cut gears; (helix angle=alpha):
Fa = Ft*sin(alpha) = (Teng/L)*sin(alpha)
(For straight cut gears; alpha = 0 => Fa = 0)
This is the axial force acting in axial direction on 1st motion shaft towards the main shaft. At the same time it is the axial force (from this gearmesh) acting on the layshaft towards the end of the gearbox. When it comes to the layshaft, it also comes an axial force contribution from the second engaged gear pair. Continue setting up a torque equilibrium equation for the layshaft with the engaged gear pairs (2nd or 3rd) and you will figure out the resulting axial force and in what direction it goes.

 

I agree.

 

I went through this with my slide-rule yesterday evening, but it was when mulling it over in bed that it dawned on me that all we need to do is to look at the gear ratios in the Manual.

 

The standard Mini 2nd gear ratio is 2.172:1 and the Cooper S is 1.916:1, so lets call it 2:1.  This give 50% more lateral bias than a 1:1 ratio and as the forces are equal and opposite, there is only 25% of the lateral force on the centre web than the 100% there would have been, if the first motion gears were straight cut.

 

A better outcome than you had thought but not quite as good as I had assumed.  All told, interesting exercise, thank you for bringing it up.

[Java_Green]

 

I have also considered using the mini spares heavy duty helical gearset. However, my main concern is about the gearbox housing when putting a lot of torque through the gearbox. Straight cut gears do not produce force transfer losses ending up as axial loads and in forth gear it should be fairly quiet as the load goes straight through the mainshaft. Worst case, using helical gears, should be in second gear during "drive" (opposite to "coast") where the layshaft is pushed towards the centre web of the gearbox housing. (At this very moment I do not have a gearbox at hand to measure the different gear radius to calculate the actual axial force level based on typical torque figures) I guess second gear is a gear that is most likely to be used when using full engine torque...

 

Just me thinking....

Not sure for others, but I rarely use full throttle in second. Mainly 3rd and 4th. I love the maths to this, a few people at work have crunched the numbers and it isn't as bad as i first thought. My other consideration of straight cut was the numerous gearboxes that have had the centre web split due to the gears trying to climb over each other at high torque loads. The evo helical sets are in a lot of high powered turbo A series engine set ups now. I know of at least 2 now running over 250lbft for a few years now. Time will tell on the cases, and bearings, as to the longevity!

 

250lbft is a lot to put through a mini gearbox.... You mentioned a reason for the centre web cracking. Is it true that it is due to using straight cut gears? I cannot see why the evolution heavy duty gearset would spare the centre web, somebody who can explain? I cannot really see why. Is it still true using a pinion support?

[Steve220]

 

 

I have also considered using the mini spares heavy duty helical gearset. However, my main concern is about the gearbox housing when putting a lot of torque through the gearbox. Straight cut gears do not produce force transfer losses ending up as axial loads and in forth gear it should be fairly quiet as the load goes straight through the mainshaft. Worst case, using helical gears, should be in second gear during "drive" (opposite to "coast") where the layshaft is pushed towards the centre web of the gearbox housing. (At this very moment I do not have a gearbox at hand to measure the different gear radius to calculate the actual axial force level based on typical torque figures) I guess second gear is a gear that is most likely to be used when using full engine torque...

 

Just me thinking....

Not sure for others, but I rarely use full throttle in second. Mainly 3rd and 4th. I love the maths to this, a few people at work have crunched the numbers and it isn't as bad as i first thought. My other consideration of straight cut was the numerous gearboxes that have had the centre web split due to the gears trying to climb over each other at high torque loads. The evo helical sets are in a lot of high powered turbo A series engine set ups now. I know of at least 2 now running over 250lbft for a few years now. Time will tell on the cases, and bearings, as to the longevity!

 

250lbft is a lot to put through a mini gearbox.... You mentioned a reason for the centre web cracking. Is it true that it is due to using straight cut gears? I cannot see why the evolution heavy duty gearset would spare the centre web, somebody who can explain? I cannot really see why. Is it still true using a pinion support?

 

It's due to the opposing forces on the shafts - If leverage is the same, then load can't be added from elsewhere. In that manner helical transfers some of the load through another axis. Force = Blue + Red

 

 

Imagine this as straight cut, all the forces will all be in the blue direction, therefore Force = Blue

 

Ive not seen a web split on a helical set yet, however that's not to say it isn't possible.

Edited by Steve220, 09 January 2024 - 09:15 PM.

[mini13]

thats fine, but straight cut gear kits are measurably less lossy than a helical kit arn't they??

[DeadSquare]

that's fine, but straight cut gear kits are measurably less lossy than a helical kit aren't they??

Does Mr. Vizard mention any trials ?

 

Helicals are stronger, because the part of the tooth, either side of the part that is engaged, is adding support.

 

The above statement is not absolute.  It depends on the angle of the helical cut and the width of the gear, but is true for a Mini gearbox.

Edited by DeadSquare, 10 January 2024 - 12:37 PM.