Wanted helical gear set for model engine

[stonewall]

looking for a set of 2:1 ratio gears that are 90 degrees to each other, that will fit a 1/2" dia crankshaft, and 3/8" dia camshaft
thanks

 

[LaFrance4]

Hey Stonewall, Go to McMaster-Carr They should have what you need.

 

[GreenTwin]

I looked for helical gears at various places, but it seemed that to get a 2:1 ratio, they used 2:1 gear diameters (check me on that).

At any rate, JasonB created a 3D model of some helical gears that are the same diameter, but give a 2:1 ratio, which is a bit of a trick to work out in geometry and in 3D modeling.

These can be scaled to about any size.

I 3D printed a set of these, and they mesh perfectly.

A bit tricky to cast, but it can be done.

.

 

[fltenwheeler]

Check with Debolt Machine

Tim

 

[Jasonb]

The shaft diameters are not so critical as you can usually bore out or sleeve the gears. What matters is the PCD of the gears (shaft ctrs) and the amount of room you have around them. Stock 2:1 gears can be had from various places but they will not be the same diameter which is what is used on a lot of engines, More detail or at least what the engine is would help

 

[Drawfiler]

Jasonb is right, the od of the meshing is critical hence the centre distances. I have made steel 2:2:1. 90* pairs for small engines so send me a pm if I can help.

 

[kuhncw]

Stonewall,

PM sent.

Chuck

 

[kvom]

I found some gears for a project here: Small Mechanical Components: Precision Gears, Timing Belts, Gear Assemblies, Timing Belt Pulleys, Couplings, Bearings and much more from SDP/SI

 

[GreenTwin]

I found some gears for a project here: Small Mechanical Components: Precision Gears, Timing Belts, Gear Assemblies, Timing Belt Pulleys, Couplings, Bearings and much more from SDP/SI

The Tech Info sheet at this site for Helical Gears indicates that the number of teeth on each gear determine the speed ratio.
This does hold for the gear that JasonB sent me, which is 20 teeth:10 teeth, or a 2:1 speed ratio.

I was a bit confused about that, since you can have helical gears of different diameters and the same helical angle, or helical gears of the same diameter with different helical angles.

The standard inch-crossed helical gears offered by McMaster-Carr are all the same helix angle, which is 45 degrees.
From McMaster-Carr:

Crossed helical gears, also known as screw gears, can be configured to transmit motion at a 90° angle.
To transmit motion at a 90° angle, pair two gears with the same tooth direction.

So it appears that if you use the standard McMaster-Carr helical gears, you can only vary the speed by varying the number of teeth, which varies the gear diameters, ie: one gear will be larger than the other.

Looking at the SDP/SI link above, it appears that the standard inch gears all have a 45 degree helix angle, and so you could not use two of them of the same diameter to get a 2:1 speed ratio (same situation as the helical gears offered from McMaster-Carr).

I have never seen helical gears of the same diameter but different helical angles offered for sale anywhere, and have not seen a commercial site that shows the calculations for this configuration.

.

 

[Jasonb]

De Bolt sell non 45deg pairs as do a few other engine suppliers

The biggest problem with using stock 45deg gears on a hit & miss engine is that the shaft gear ends up being quite large so has to stick way out the side of the engine. Or if a 4-stroke aero engine then the gear housings are usually too small

tooth count is the only thing that determines speed ( ratio ), angles change the diameter and therefore PCD

 

[GreenTwin]

angles change the diameter

I am not sure I follow you on that one.

The diameter of two gears can be the same, while the angles can change to a wide range of values?

I am still confused about the angles vs the tooth count.
If the speed ratio is only related to tooth count, then why does our 2:1 speed ratio gear example above have to use roughly a 30/60 degree tooth angle?

I have looked at same-diameter helical gears for a while, and I still can't put into words exactly what is happening to cause the 2:1 speed ratio, along with a clear explanation of how that relates to the varying angles for each gear.
I see what was done with the gears that I 3D printed, but I don't know why it works.

It seems like for same diameter gears with a 2:1 speed ratio, the angles would have to be 30 and 60 degrees exactly, but they say it is not so.
.

 

[mauryuebelhor]

GreenTwin, I am working on this same problem for an engine kit I have that did not include the timing gears.

You neglected to give a very critical piece of information: the c/c shaft spacing. This number will be equal to the sum of the pitch diameters (Pd) for the pinion and the gear.
The Pd for a helical gear is : Pd= N/DP * Cos(Ha)
where N= Number of teeth
Ha = the helix angle.

This equation holds for spur gears as well, as the Cos(0)=1 so, it just cancels out.
To solve for the pinion:
If you solve the equation for Cos (Ha) you can just plug in the numbers you want, and take the ArcCos of the result to get the helix angle. Note, there is a range of values that work.

Next if you subtract the Ha from 90 deg, and again solve the equation for Pd, substituting the values in, you get the Pd for the gear.

For gears of the same Pd and a ratio of 2:1, you will find the pinion Ha =63 deg 20 min, and for the gear, the Ha =26deg 40 min.

Hope this helps
maury

 

[GreenTwin]

Maury-

Someone sent me a spreadsheet for helical gears; perhaps it was you?

I forgot that I had that, but I will go find it and compare it with your formulas.

The math should be easy for me, but unfortunately it is not.
While I understand math pretty well, and I can understand spur gears and their equations pretty well, when things go polar, I tend to lose the "feel" for what is going on.

Just like I can understand a steam engine valve travel sinusoidal displacement diagram much better than a polar diagram.

But it seems like you sum it up rather well, and in a simple fashion, so I just have to wrap my head around it, and then put it into my own words.

Thanks very much for that explanation.
I have been scratching my head on this for months.

Pat J

 

[Jasonb]

As the diameter of the gear (pcd) is calculated as number of teeth divided by (DP x cos helix angle) if you want to keep the diameters the same then the helix angle will need to be different for each of the two gears as you can't change the number(ratio) of teeth or DP.

Taking a 10:20 tooth pair 20dp

If you buy stock 45deg gears then the diameter of the larger tooth count one will be bigger than the smaller

10/20 x cos45 = 10/20 x 0.707 = 0.707" for the 10tooth and 20/20 x cos 45 = 20/20 x 0.707 = 1.414" dia for the 20tooth

Now doing the same for equal diameters we cant change the DP or number of teeth so the angle has to change

10/20 x cos 63.43 = 10/20 x 0..447= 1.118"" dia for the 10tooth and 20/20 x cos 26.57 = 20/20 x 0.894 = 1.118" dia for the 20tooth

Easy way to get the helix angles for a 1:2 pair of the same diameter is draw a right angle triangle in CAD 1 unit on the short side and 2 units on the long and measure the angles

 

[Drawfiler]

Greentwin
lets keep it simple, the ratio (in this case 2:1) is determined by the tooth counts irrespective of whether the gears are straight spur or helical.
when the gear shafts are not parallel the diameter of the gears will change but the ratio is still dictated by the relative tooth counts.
to cut correct form helicals without a hobber you will will quite a sophisticated set up, by milling them with the leadscrew linked to the dividing head.

 

[GreenTwin]

GreenTwin, I am working on this same problem for an engine kit I have that did not include the timing gears.

You neglected to give a very critical piece of information: the c/c shaft spacing. This number will be equal to the sum of the pitch diameters (Pd) for the pinion and the gear.
The Pd for a helical gear is : Pd= N/DP * Cos(Ha)
where N= Number of teeth
Ha = the helix angle.

This equation holds for spur gears as well, as the Cos(0)=1 so, it just cancels out.
To solve for the pinion:
If you solve the equation for Cos (Ha) you can just plug in the numbers you want, and take the ArcCos of the result to get the helix angle. Note, there is a range of values that work.

Next if you subtract the Ha from 90 deg, and again solve the equation for Pd, substituting the values in, you get the Pd for the gear.

For gears of the same Pd and a ratio of 2:1, you will find the pinion Ha =63 deg 20 min, and for the gear, the Ha =26deg 40 min.

Hope this helps
maury

I had a brain freeze this morning, but that name "Maury" seemed rather familiar.
I had a lot going on this morning, with truck deliveries, and all sorts of things.
Now I realize who "Maury" is, LOL, I know this person well and have followed their fabulous work for years.

Do you guys know who this "Maury" is ?
I do.

Thanks Maury for the feedback.
Much appreciated, and glad to hear from you.
Its been a while. What are you up to these days?

Pat Jorgensen

.

 

[GreenTwin]

Greentwin
lets keep it simple, the ratio (in this case 2:1) is determined by the tooth counts irrespective of whether the gears are straight spur or helical.
when the gear shafts are not parallel the diameter of the gears will change but the ratio is still dictated by the relative tooth counts.
to cut correct form helicals without a hobber you will will quite a sophisticated set up, by milling them with the leadscrew linked to the dividing head.

Thanks for the input.

"when the gear shafts are not parallel, the diameter of the gears will change, but the ration is still dictated by the relative tooth counts".

Except for the gears on the Frisco Standard marine engine gears, which are the same diameter, with shafts at 90 degrees, and a 2:1 speed ratio.
So for the Frisco Standard gears, the gear shafts are not parallel, but the diameter of the gears does not change.
The speed ratio does seem to be dictated by tooth count.

I am going to cast the gears JasonB sent in gray iron, using a modified lost-pla process.
I will have to figure out the exact center-to-center distance I need, and then add in the correct shrinkage factor.

The backlash will be adjusted by using oversized holes in the bearing housing, so that it can be moved slightly closer or further away from the crankshaft.

.

 

[GreenTwin]

Spur gears are pretty easy to understand in general.

I have seen old wood gears on Dutch windmills, and they ofter are just pegs mounted in a flat disk.
The fit on the old wood gears was not critical, and so there was sufficient clearance for the gear teeth to clear each other as the gears rotated.

The Archimedes screw was used in ancient times to pump water, and while watching a bare screw move, it visually appears that the helical parts of the screw are moving linearly down the length of the screw.
Of course the helical spiral of a screw is fixed to the shaft, but the effect is like a wood screw, where if you turn the shaft, the screw progresses into the material.

The rate at which a screw progresses into the material is related to the angle of the helix.
Quick screws (not sure of the exact term for them) can bottom out in as fast as a half turn or less, and they have a very high angle on the helix.

Fine threaded machine screws progress very slowly as you turn them.

For spur gears, in order to increase or decrease speeds, one can use a smaller gear to drive a larger gear, with the speed ratio determined by the tooth ratio between the large and small gear. The trick is that the teeth have to be the same size and general shape on both the smaller gear and the larger gear.

Crossed helical gears and worm gears seem very similar, and seem to operate about the same way.

I think worm gears are generally designed to provide high torque to the shaft which has the larger gear, and they are not designed to run backwards, ie; the large gear cannot power the small gear.

Crossed helical gears do not adhere to the same rules as spur gears, ie; for varying speed ratios (such as 2:1), the diameter of the two helical gears can be the same, as long as the angle on the teeth of the two gears is different.
You can use helical gears of different diameters to get a speed ratio, but you can also use helical gears of the same diameter to get different speed ratios.

All of the commercial helical gears I have seen have a fixed angle, such as 45 degrees, and to get varying shaft speeds, you need gears of different diameters.

I was baffled when I saw the first crossed helical gears on a side-shaft IC engine that were the same diameter, but running at a 2:1 speed ratio.
"How does that work?" I remember saying.

Orientation of the gears and gear angles for helical and maybe some other gear types can be critical depending on whether the shafts will be parallel or at an angle such as 90 degrees.

It surprises me that when gear teeth are spiraled around a shaft, they will mesh with each other, especially with crossed helical gears.
The reason crossed helical gear teeth mesh I think is because the perpendicular section of the teeth on both gears is identical, even though the angle of the spiral differs.

Crossed helical gears I think are not used in high-power applications, because the tooth contact is a point (check me on that).
A spur gear tooth contact area would be a line.
I think helical gears used on parallel shafts will transmit large amounts of power very smoothly.

So as JasonB says, when we make crossed helical gears, the only variable that is not fixed is the angles of the teeth on the two respective gears.
If we fix the tooth angle to 45 degrees, then the only way to have a differential shaft speed is to have gears of different diameters.

If we use Maury's Ha =63 deg 20 min, and for the gear, the Ha =26deg 40 min, with crossed helical gears of the same diameter, those angles are the only thing that can vary (as Jason says).

The crossed helical gear speed ratio is always determined by the tooth count, but if you don't get the angles correct, then I guess the teeth don't mesh as they rotate at their 2:1 ratio.

The gear with the lower tooth angle goes on the crankshaft, and it rotates twice for each rotation of the gear with the higher angle.
Both gears have to have the spiral going the same way (either both clockwise or both counterclockwise) when looking at a section of the gears.

If you think in terms of wood screws, the gear with the lower tooth angle will progress into a piece of wood at half the speed as the gear with the higher angle.

The 2:1 ratio of the Frisco Standard gears is due to the 2:1 tooth ratio, but also seems to be due to the two different tooth angles, since it takes both factors to produce the 2:1 speed ratio.

If the speed ratio of crossed helical gears was only due to the 2:1 tooth ratio, then you could have a 45 degree tooth angle on both gears.

Edit:
I think the reason crossed helical gears mesh, when the same situation would not work on two of the same diameter spur gears is that you sacrifice a large tooth contact area, and have only a point contact with crossed helical gears.
That is how crossed helical gears can apparently magically mesh when conventional wisdom would seem to indicate that they should not.
.

 

[Jasonb]

The 2:1 ratio of the Frisco Standard gears is due to the 2:1 tooth ratio, but also seems to be due to the two different tooth angles, since it takes both factors to produce the 2:1 speed ratio.

If the speed ratio of crossed helical gears was only due to the 2:1 tooth ratio, then you could have a 45 degree tooth angle on both gears.

.

It's only the tooth counts that determine the ratio/speed NOTHING to do with the angles

Angles only alter the diameter of the gear.

Two reasons we alter the angles on our models are

1 To get the crankshaft gear with it's small number of teeth to fit around the crankshaft without the bore being so large that it weakens the teeth.

2 To get the camshaft/sideshaft closer to the crankshaft either to reduce the room needed for the higher tooth count gear and keep the shaft closer to the ctr line of the head and /or to save having the flywheel further out which puts strain on the bearing and crankshaft.

 

[GreenTwin]

I can see the need to keep the sideshaft close to the engine, othewise you would have some potentially large cantilever supports for the shaft, and long rocker arms to get from the cam back to the valves.

And the power required to lift the valves is minimal, so the crossed helical gears of the same diameter can carry the load easily.

It's only the tooth counts that determine the ratio/speed NOTHING to do with the angles
Angles only alter the diameter of the gear.

I think I can see that statement as being true.

I think it can be also said that for two crossed helical gears that have teeth at the same angle (say 45 degrees), using gears of different diameters and tooth counts is the only way to vary the speed of the two shafts.

And it seems in every case that the tooth section has to be the same, else the gears will not mesh.

Crossed helical gears of the same diameter seem to be a bit of an unusual design and application.
It was a twisted mind that came up with that design (no pun intended).

Edit:
So I guess it begs the question, could two crossed helical gears be made of any varying diameter, and the 2:1 ratio be held by varying the tooth angle proportionally?
For example one gear is 1.5" diameter, and one gear is 2.5" diameter, and they have a 2:1 speed ratio.

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