Helical Gear

[M-man]

How does it work? I ve got 2 helical gears mounted in 90degrees angle.

On gear got od 31.1mm 18 teeth, helical angel 50, (lead 104.79mm)

other gear got od 40.7mm 18 teeth, helical angel 40 (lead 97.49mm)

I know that helical angel 40+50 = 90 so that prob why they work togheter, but how do they get the profile of the tooth, they looks like the same on booth gears and the got the same amount of theeth, normaly a bigger gear would have the same modul but more theeth to match.
Does anyone know any thing about this?

I did cut my gears both in 45 degrees angel, but I did have to use 18 theeth and 22 on the other to match the gears. But I would really want to cut them so I will get 18 theeth on both gears as the original parts have.

[NC Cams]

An answer to your inquiry can be found in the "bevel, worm and helical gear" chapter of:

"Design of Machine Elements" by M.F. Spotts

Mine is quite dated (circa 1961) but the formulae do not change. The formulae you need to solve your problem are found in the listed reference. THe math involved is a bit beyond what can be readily conveyed via a message board.

The tooth angles are purely a function of pitch and the included angle between the shaft

Gear tooth count is a function of shaft-to-shaft distance (ergo the pitch) in concert with the desired ratio.

Add some math and cutter magic and instant gears.

[HuFlungDung]

My question would be why did you cut both gears with a 45 degree helix angle when you knew the originals were 40 and 50?

It is true that the normal profile is almost identical for both the small and the larger gear, particularly if they have the same number of teeth. By 'normal' profile, I mean the cross section of the tooth measured perpendicular to the tooth's face.

For a given diametral pitch (or module), the circular pitch of the teeth on a straight cut spur gear is the minimum circular pitch possible. This straight tooth spur gear also has the maximum number of gear teeth possible for a given pitch diameter.

As soon as you add in a helical twist to the tooth, the circular pitch gets larger, and the tooth number on the same pitch diameter must decrease, because you have increased the effective width of the tooth as viewed on the side of the gear. Its a case of 'only so much circumference available', and if you increase the circular pitch of the tooth, fewer teeth can fit in the allowed space.

So that is why the original gears could have different diameters and different helix angles and yet have the same number of teeth.

The short of it is, you cannot cut both gears with the same setup. You need to tear the gear train down because you need to change the lead for each gear.

If the lead figures you derived at the start are correct, then that is how you should gear up the dividing head. I'm not saying that your figures are correct, it would take me some time to figure it out, even having half an idea about what is going on.

[M-man]

I thougt the gear should be 45 degrees coz they were mounted in a 90 degrees angel. It were after comparing to the original part I discovered that they had used one 40 and one 50 degrees angel.
I uses the lead calculated from S=Pi*D*cot a, so I uses 2 different setups,( I actually cuts the gears in the lathe)(with a bit of fantasy most stuff works).
The gears I machined actually works, they will have the wrong ratio, but that doesnt matter for what I am going to use them to.
I am going to try cutting a new set of gears with 40 and 50 deegres helic angle, this I think will produce a thicker rooth on the smaller gear and a thinner thooth on the bigger gear so I dont have to add 4 theeth.

Is there any way to get the profile of the MODUL 2 for a 18 theeth gear in to a DXF?

The pics shows the original part, but helix goes clockwise and I want to have one that gesa anticlockwise. In one of the pics you can se that the angels doesnt match up, so 5 degrees wrong on my gears. I cut them in alu coz it goes alot faster, when or if i get a proper set of gears I will cut them in steel.

[NC Cams]

Classic distributor drive gear setup in an automotive application.

Are you sure it is module instead of diametral pitch?

Do you have the TRUE center distances and the helix angles figured out?

Keep in mind that SOME automotive gears use custom, non-standard pitches for unique center-to-center packaging distances - this necessitates custom cutters.

The high helix angles used on these gears causes REAL problems whenever we try to have gears cut - very few custom gear cutting shops can cut them due to the helix angles.

You can't simply change anything to suit what you can make due to the packaging constraints (IE ratio, pitch and center distance, helix angles, etc).

I wouldn't get too creative here.

[M-man]

I am going to be creative, the gear will only run the oilpump and not any distributor so the ratio isnt so important..

I finally got hold of a exell prg that calculates gear profiles and creates them in DXF for me. So now I kan produce the proper tool profile.

I was planing of cutting the gear in SS1672, wich I will harden to about 54-55 HRc. The maximum speed the gear will have is around 4000 RMP, the load of the gear should not be so heavy.(At least I think). Hopefully this will work.

[NC Cams]

I strongly disagree.

You do NOT want to be creative here. The speed ratio IS important.

Why?

The distributor is typically running at 2X cam speed. This is why the cam gear is usually 2X the tooth count of the distributor, namely to speed the driven gear back up to crank speed - and also to speed up the oil pump too.

Unless you've drastically increased the output capacity of the pump (IE: high volume), any speed REDUCTION in the pump WILL result in a proportional REDUCTION of oil flow to the engine.

BTW, bronze distributor gears don't live long/well in general automotive use. Reason: the relatively harder mesign gear tends to cut them up/wear them out prematurely. Moreover, the typical machining tolerances in the block result in poor tooth match which further aggravates the wear issue.

The 54-55 Hrc hardness is "adaquate" as a number number. HOwever, it may not be optimum for the gear set. On a gear set where you have a 2:1 ratio, the smaller tootch count gear is often made a bit harder than the larger one.

Why?

It has 2X the tooth meshes per revolution and thus 2X the wear potential. It doesn't take much of an Hrc differential - the smaller gear only needs to be a couple points HARDER than the large gear to help even out wear potential.

Caution: Creativity and/or incorrect assumptions ( "...At least I think..." ) in automotive gear trains can create copious problems.

[Chris64]

Those gears look pretty cool to me. I'm not quite sure if this was mentioned because I didn't understand a lot of what was said...but my concern with this would be resistance. It would seem from looking at it that the reason for going 50 + 40 degree's would be to increase the efficiency of the gears (from one to another). In an extreme angle this would be like a worm drive which would make it easy for gear "A" to drive gear "B" and impossible for gear "B" to drive "A". So if they are both at 45 degree's, you may experience excessive resistance or wear (or feedback, but probably not in this case).

If it works, it works.

[M-man]

I had some luck with the 50 and 40 deegres angels today. As calculated I managed to produce 18 teeth on both gears, but with different OD, and it all were cut with the same tools, mounted in different angels.The profile were not perfect but good enough for me.
So I tried to cut the gears in steel, disaster, my tool broke after cutting the third tooth. ha ha... seems like I wasted some time...

distributor - Who has one?

[NC Cams]

The helix angle of the gears can NOT be arbitrarily changed - it is part of the many issues that the original drive designer used to package the gears within the space given at the original ratio/tooth count/pitch.

The reason the OEM used a 50 and 40 P/A was an integral part of the original ratio and the rest of the variables involved to create properly meshing gears. How do we know? We've already tried to go down the path you're trying to go down and found it to be a disappointing dead end.

Improper pitch, lead, backlash and helix angle will result in the gears essentially "eating" each other as they try to get each other to mate. Sadly, this won't happen as you'll run out of gear teeth material before they get each other to mate/mesh properly. That or else you end up with some screwed up ratio that isn' close to what you want/need.

If you don't care about form, fit and long term function of the system, proceed with SWAG"s as you are. Otherwise, you might want to do a bit more research into the how's and why's the helix angles and gear specs were chosen for this particular drive by the OEM for the given system.

You'll find that you won't waste anymore time or anymore cutters nor will you make a drive that just may not do what you want/need it to do.

The Spotts book or any other helical gear design manual will give more insight into solving the problems that you face and more importantly, why it needs to be done that way instead of one that is merly "good enough for me".

[M-man]

If I copy the gear profiles and knows the helix angel, It should work without spending 2 years reading about it. Today I grinded a new tool according to the DXF file I ve got, the profile seems to be the correct for both gears, so I will try to cut some more gears.I designed the tool a bit stronger this time, and I think I found out why the old tool broke.. So I am going to try some more..

[NC Cams]

To recap:

" How does it work? I ve got 2 helical gears mounted in 90degrees angle.

On gear got od 31.1mm 18 teeth, helical angel 50, (lead 104.79mm)

other gear got od 40.7mm 18 teeth, helical angel 40 (lead 97.49mm)"

A: You then reportedly averaged out the helix angle for an unexplained reason, cut the gears and found they didn't work.

B. You then reportedly changed to match the helix angles and got gears that were the wrong OD's but he right tooth count.

C. You then reportedly made another change (material I believe) and then broke the cuter all the while getting gears "close enough for me" but still not idendical to the OEM's if I understand your plight correctly.

In light of the fact that we make gears regularly like you are trying to make regularly for the billet cams we make and sell, we do have a bit of expertise in the art and science of distributor and cam gear cutting.

If you care to learn, the solution to your problem are outlined in posts #3 above with backgound explanation outlined in post #2. Moreover and more importantly, it doesn't take 2 years of reading to solve your problem. However, it does take a small amount of effort to understand and comply with the truly well documented art and science of helical gear design and cutting.

Frankly, you could have read the chapter on gear design in Spotts (or any other helical gear design manual) and learned what helix, tooth diameter width and all the other stuff mean and why they are important to cuting gears for a gear train that meshes/performs properly.

Moreover, you could have done it (the reading) in less time than you've spent cutting gears that STILL don't work, which surely would be less than 2 years in duration.

BTW: it isn't just the profile that has to be right - the right profile and the wrong lead, wrong tooth width, and/or wrong root/OD/PD will cause gears with other stuff that is "close enough for me" to cause the gears not to work.

The OD on gears is NOT a function critical dimension - it is typically listed for REFERENCE only. You can NOT use it to properly size/cut gear teeth or quantify profile or PD correctness

Final hint: the same cutter can NOT be used to cut distributor gears like you're trying to cut for reasons outlined in post #3. The explanation behind the facts cited immediately above and those put forth in post #3 can be found in Spotts.