Outer Limits 4x4 Board

Has anyone had gears cut? Just after an idea of a fair price and how they turned out.

i am not sure how much you allready know, so here goes.
i have not had gears cut but i can tell you this.
Gear tooth profile design is a field of engineering of its own. if your shafts have to fit into an existing case ie. transfer case, the distance between the shafts will be fixed.
There are a series of standard gear tooth profiles.
for a tooth profile, you can increase the "diameter" of the gear in 1 tooth incriments.
therefor to make a set of gears to fit into an existing housing, you would have to fluke that you can find a standard tooth profile that a)gives you the desired ratio and b)matches the shaft centre distances determined by your housing.
you can buy many standard gears off the shelf and the prices are quite reasonable.
if you cannot find a standard gear you would have to have a tooth profile designed for you and custom cut. i reackon this would cost quite a few thousand dollars.
hope this has helped.

Frank (Bitsamissin) organised custom low range gears to be cut for 3Litre Pajeros. They were done at Hardman Bros. (who apparently do some gear cutting for Air Lockers) There were 20 sets cut and a price arranged on that amount. They are currently looking at doing more for the 3.5 Litre Paj. I don't know how you would go getting a single set done. Do a search in the Mistubishi forum on Transfer gears and you should turn up some info.

I know a bloke (engineer) who made his own gearbox for his tractor pull tractor that has 1000+ HP. It has different ratios but you have to pull the box apart to change them. It's only a 10 minute job for him.
I dont know how much it would cost to do them tho. I can get his number if you want. Southern NSW.

85lux is partly correct.

The common gear tooth profiles are usually 20 degree pressure angle but could also be 25 deg (for more strength) or 14 degree (quieter but not so common because of lower stength and cutting accuracy is better these days). For helical gears the pressure angle will either be normal or transverse depending upon what kind of cutter (rack or hob) is used.

The tooth sizes are either based upon DP (diametral pitch - imperial) or Module (metric). DP is number of teeth per inch of pitch circle diameter. Module is tooth adendum (height of tooth above pitch circle) in mm. You don't find non standard tooth sizes - always based on available standard cutters.

Helix angle for helical gears is not standardised if teeth are hobbed but is restricted by available cutters if gear cutting machine uses rack cutters.

Addendum correction is what 85lux was referring to. But it does not necessarilly increase in 1 tooth increments. The ammount of addendum correction is not standardised. It is usual to use positive addendum correction on pinions with a small number of teeth to avoid tooth undercut. An undercut tooth is weaker than one with positive addendum correction.

If the pinion has posititive addendum correction and the wheel has the same amount of negative correction, the centre distance will be standard. If you need a non-standard centre distance then the addendum correction could be varied either way, within practical limits. Usually about 1 tooth diameter bigger in diameter is practical.

Some gears require tooth profile correction, to prevent iterferance between the tip of one gear and the flank of the mating gear, due to deflection of the tooth under load.

With large gears (not in your case) the pinion teeth may need to be cambered to compensate for bending deflections of the shaft. Helix correction may also be used for wide gears to compensate for twisting.

Hardness is the most critical material property for tooth strength. It also greatly affects how the gear is cut and heat treated. Some gears may be through hardened and the gear teeth ground (using a gear grinding machine) after hardening.

Case hardening is also common. The case depth is dictated by the tooth size.

One advantage of case hardening is the teeth can be hardened after the gear cutting is completed. EN39B is a case hardening steel with tremendous properties of core strength and hardness for very high strength gears, but is a bitch to work with.

I don't know if this helps, but you need to realise that gear design involves many variable. Just because some shop has a gear cutting machine, does not mean the will be able to cut a particular gear, because some gear tooth profiles may be dependant on the type of cutters used.

Gears should be designed as pairs. A pinion with 19 teeth that mates with a 30 tooth wheel is likely to be different to another 19 tooth pinion that mates with a 100 tooth wheel.

I forgot to mention "gear quality number" in the last post.

The quality number specifies the allowable tooth profile error and pitch error. The allowable errors are usually very, very small. Numbers above 12 usually need gear grinding machines to finish the tooth profile.

A higher number will give stronger teeth and quieter running, but is harder to achive. Some types of gear cutting machines are better than other for producing higher quality numbers.

Now thats some serious tech....... well done Bush65

thanks for correcting me there. obviously you have worked with gears a bit before. good post.

John, outstanding reply

I have been looking around and found many shops vague to my questions. Mostly I have been getting yes or no answers (when they bother to answer). As for hardening, a few have different ideas with so doing it after the hobbing, some just doing the teeth but none mentioned doing it before the hobbing. Is this just because it is a pain in the butt?

There seems to be a few shops about but not many at all with successful automotive experiance.

Nev

Nev62 wrote:John, outstanding reply

I have been looking around and found many shops vague to my questions. Mostly I have been getting yes or no answers (when they bother to answer). As for hardening, a few have different ideas with so doing it after the hobbing, some just doing the teeth but none mentioned doing it before the hobbing. Is this just because it is a pain in the butt?

There seems to be a few shops about but not many at all with successful automotive experiance.

Nev

Gear cutting and designing (or reverse engineering) gears are 2 different fields. Some larger places are able to do both.

As I said before, gears should be designed/made as pairs. Cutting a new gear to mate with an existing gear can lead to lots of problems if you don't have the original tooth data.

Accuracy is vital to gear strength and operating noise. They would lead themselves open if the gear failed or was noisy operating with a used gear.

Gear teeth do not wear uniformly. The profile changes because the contact pressure between mating teeth varies from the tip to the root as go into and out of mesh. Also the action is rolling near the pitch line but sliding near the tip and root.

Some larger places could do heat treatment, but many could not. There can be problems with distortion during heat treatment, depending on the type of material, the method etc. There are not many gear grinding machines in Australia for finishing gears after heat treatment.

Many shops would only cut gears which have a bore that will fit over the spindle of their gear cutting machine. And a hobbing machine needs plenty of room on both sides of the gear, for the hob to approach and depart the gear blank.

I don't think automotive use has much influence on the gear cutting process, except the gears are often through hardened and ground. Reverse engineering would be a problem because they are much more likely to have addendum correction and non standard centre distance.

Hi Guys
Interesting to hear the tec on gear cutting. Ser 111 gearbox is capable of handling about 90bhp. This is a big drawback on engine transplants. Apart from bearing problems teeth havebeen stripped regularly on tdi engine conversions. Is there any way that gearbox could be viably strengthened on rebuild. I`ve often wondered about an aftermarket specialist supplying stronger gears for that box. Is it possible engineering wise or just not feasible commercial wise. I think the market would be there in UK alone but then that box dates back to the 50s and that speaks for itself.
Cheers Alex

lexi wrote:Hi Guys
Interesting to hear the tec on gear cutting. Ser 111 gearbox is capable of handling about 90bhp. This is a big drawback on engine transplants. Apart from bearing problems teeth havebeen stripped regularly on tdi engine conversions. Is there any way that gearbox could be viably strengthened on rebuild. I`ve often wondered about an aftermarket specialist supplying stronger gears for that box. Is it possible engineering wise or just not feasible commercial wise. I think the market would be there in UK alone but then that box dates back to the 50s and that speaks for itself.
Cheers Alex

I think the cost would be prohibitive.

lexi wrote:Hi Guys
Interesting to hear the tec on gear cutting. Ser 111 gearbox is capable of handling about 90bhp. This is a big drawback on engine transplants. Apart from bearing problems teeth havebeen stripped regularly on tdi engine conversions. Is there any way that gearbox could be viably strengthened on rebuild. I`ve often wondered about an aftermarket specialist supplying stronger gears for that box. Is it possible engineering wise or just not feasible commercial wise. I think the market would be there in UK alone but then that box dates back to the 50s and that speaks for itself.
Cheers Alex

As Bush said above the cost would be prohibitive. You cannot even get genuine gears for the serie boxes anymore - so land-rover don't even think the market is viable. It would likely be far cheaper to install an R380 with the ashcroft conversion to mate an R380 to a series T-case.

I am assuming most of this thread is concerned with syncro gears! I'm just wondering what people, in particular John think about moving to a straight cut box? Would this be just as hard to do? Despite the obvious drawbacks, would there be any strengh, or ratio gains yo be had??

Thanks, Richard.

blackhj wrote:I am assuming most of this thread is concerned with syncro gears! I'm just wondering what people, in particular John think about moving to a straight cut box? Would this be just as hard to do? Despite the obvious drawbacks, would there be any strengh, or ratio gains yo be had??

Thanks, Richard.

Do you mean a non-synchro box or a straight cut box? You can still have helically cut gears in a non-synchro box, you just need the gears to be in constant mesh and a dog to lock the gear to the shaft to engage it.

I'm sure John will correct me if I am wrong, but helical gears should be stronger (and quieter) than straight cut gears of the same ratio and same gear width, but generate more heat because of the sliding action.

If you wanted to get a complete non-synchro box, it would probably be cheaper to buy a hollinger 6-speed ($16k) than get the gears made. Cheaper again would be to find a suitable truck gearbox, but the modern ones small enough to be used in a 4x4 are synchro.

Surely if they're making more heat they're sapping more power too?

Strength depends on many variables- you cant just say straight cut patterns are stronger than helical without knowing contact areas, preasure angles etc. Despite what the back yarders like to claim, smaller teeth are generally stronger- greater contact area. The biggest limiting factor with automotive work is still the the synchro's. They are typically very close to the cut face of the gears- so when you cut one or the other the machine follows through and lops the top off the other face. Very few places in Australia have the expertise to do this.

Strength depends on many variables- you cant just say straight cut patterns are stronger than helical without knowing contact areas, preasure angles etc. Despite what the back yarders like to claim, smaller teeth are generally stronger- greater contact area. The biggest limiting factor with automotive work is still the the synchro's. They are typically very close to the cut face of the gears- so when you cut one or the other the machine follows through and lops the top off the other face. Very few places in Australia have the expertise to do this.

If all other factors are equal, helical gears are stronger than spur gears, given typical face widths. The increased strength is mainly due to more teeth sharing the load (the number increases with face width and helix angle) and an increase in the transverse pressure angle.

Spur gears (straight cut) are very noisy.

The main disadvantage of helical gears, compared to spur gears, is the side loads that are created. Also cutter runout as Liam mentioned. Rack gear cutter require much less space for cutter run out (and I include Fellows gear cutters with rack types). Sunderland gear cutters are the only type that can cut helical gears with no runout space, but they are resticted to 0 or 30 deg helix angle.

Heat and friction are generally not an important issue when deciding to use helical or spur gears.

The trend is to transmit larger amounts of power with smaller packages and this dictates the use of hardened and ground helical gears. Because of the smaller package (not so much the helical teeth), it is usual to require oil coolers.

I agree with most of what Liam said, except that smaller teeth are generally stronger. Not given the other factors such as pressure angle, material, heat treatment, precision etc is the same. This is assuming the the gear with larger teeth has a reasonable number of teeth (to avoid undercut) and there is sufficient marterial below the root of the teeth.