is this enough info to make a worm gear segment today? 5

[Tmoose]

presumably this was enough info to make a worm gear from the 1930 until the 1980s.
please see attached image.

A new supplier wants to know what the pressure angle is, and is confused about what we want when we say "the single thread RH helix angle of 4 degrees 46 minutes"

We are trying to locate the print for the worm.

thanks

Dan T

 

[Tmoose]

I'll see if I can dig up the arrangement drawing. Does the description on the gear drawing as "gash cut" suggest spiral or non-spiral?

 

[gearcutter]

It's possible to gash both spur and helical profiles.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[tbuelna]

I believe the term "gash" implies a hobbing process. And since the gear drawing calls out a (RH) helix angle, but no spiral angle, we can assume this is intended to be a conventional helical gear.

The Boston Gear stock part no. given is for a 14.5deg PA 8DP 1.50"PD single start cylindrical worm. So the gear teeth should probably have the same pressure angle.

While I have heard of this type of gear combination being used, I don't have any particular knowledge of how well it works. To me it seems like it would be better to use a conventional worm gear rather than a helical gear, since the worm gear would provide a more conformal/conjugate contact with the worm. Maybe someone that has some experience with this type of gear arrangement can provide an explanation of why it would be used.

 

[spigor]

tbuelna, it works provided there's very little load and the small area of surface transmitting all the power due to the point contact can handle it. Or, the worm is hard steel and the helical gear is bronze, or in worst case cast iron, so the gear can "wear in" in time with the worm and develop a bigger contact surface. You must not overload the gear until it wears in, and you end up with bigger backlash.
I saw some photos of such a set the other day. The worm was hard steel and the helical gear was soft steel. Although the load was small, each tooth on the gear had a seized spot right in the middle of it, and the worm had a seized helical line on booth teeth.
The reason to use this setup is because the axial position of the gear is not important, you can slide the gear in under the mounted worm, and you don't need to have a special hob to cut the gear, you use standard tools instead.

 

[mfgenggear]

a throated worm gear uses a hob that is slightly larger than the worm. it can be fed in radially. to the desired center distance that is required. while a helical gear is somewhat simpler to fab. it is inferior to worm gear & worm setup. and is not that much more harder to fabricate.
while the mystery has been solved. if this setup has been working & produces satisfactory results. then it is not necessary to change design.

my point about a clear and concise print is exactly why it has to be clear. just alone in this post several opinions have prevailed.
when dealing with gears it is required to have drawing that are clear and concise on what the designer requires. so there is no mistake by the manufacturer. thanks to spigor for explaning why a setup like this is used.
but to my opinion a set up like this it would be critical to obtain the correct center distance and backlash. & a break in period.

also the worm is simple to fabricate because it can be cut like a thread.

Mfgenggear

 

[Tmoose]

FWIW The 1932 vintage drawing for the bracket that includes both the worm shaft and gear shaft bores shows them as perpendicular to each other, and whopping 4-5/16" C-C / mounting distance.

The gears are for an adjustment that is made at installation and likely not changed for years.

 

[Occupant]

And that's about the clearance you'll need to use a straight sided worm gear.

 

[gearcutter]

If the mounting bores are perpendicular; then the worm's mate is NOT a "straight sided worm gear". It can only be a 4deg, 46' RH helical gear otherwise the two items will not be able to be assembled..........no matter how much clearance there is.

Tmoose: What does the following mean exactly?

"FWIW"

"4-5/16" C-C /"



Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[mfgenggear]

gear cutter is correct again. the worm is basically a rack. there for there must be conjugate action or it will not mesh correctly. the helical gear must be cut with an involute.

Mfgenggear

 

[tbuelna]

I also think gearcutter is correct. Here's what AGMA says about a straight sided profile worm thread:

"3.5.1 Straight sided axial profile -- Form ZA
This profile can be produced with a straight sided lathe tool placed on the axial plane as shown in figure 5. If a rotarymilling cutter or a grinding wheel is used to produce a ZA profile worm thread, such a cutter or
grinding wheel would require convex profiled cutting edges. Profile variations of the worm are easily checked for this FormZA profile since it is a straight line in the axial plane. The central section of the mating wormgear is theoretically an involute shape and its profile variations can be checked with an involute checker"

It seems the mating helical gear teeth should have an involute flank profile.

 

[gearcutter]

Seems to me like there's a bit of confusion that needs clearing up.

The 'straight sided' term being used in this thread comes from Occupant's thought that the gear is a spur gear (straight sided) rather than a helical gear. Occupant made this quite clear in a post above. It was not in reference to the form profile.

"I mean exactly the opposite namely that the gear is not helical. What is helical is the worm. From the gear drawing: "mesh with worm of 8DP, single thread, right hand, helical angle - 4deg. 46min. And that's exactly what the helical angle turns out to be - given the pitch and pitch dia. of the worm." - Occupant.

And this -

"And that's about the clearance you'll need to use a straight sided worm gear." - Occupant.

While it is possible to run a worm shaft with a "straight sided" or spur gear; the mounting shafts would not be perpendicular in that case. The parts could only mesh if the mounting shafts are set at the same angle as the worm's thread.

Tmoose has already confirmed that the shafts related to the original post are in fact set perpendicular to each other.

Therefore; it is misinformation to suggest that the part mating with the worm shaft is a spur gear, or "straight sided" gear.

I hope that clears things up

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[mfgenggear]

hahahaha

OK here we go
a straight sided (angle with no involute) gear has no involute.
E.G. straight sided 45 deg spline.
spur gear would not be a straight sided gear. it has an involute.
a helical gear would have to have a helix to match the worms lead angle.
a helix for a worm can be calc. like this .
90 deg - lead angle = helix angle of the worm. (Vogel)
in order for the set to mesh both gears must have the correct helix.

that is why it is important that all the pertinent data must be given.
my original statement stands.

Mfgeneggear

 

[Tmoose]

Hi Gearcutter

"FWIW" = For What It's Worth. The newsgroups and bulletin boards I frequent seem to have adopted it

"4-5/16" C-C " is the center-to-center distance (you called it mounting distance I think) between the shafts as dimensioned an the bracket drawing.
And that is about 1/16" greater than what you calculated "The correct mounting distance should be 108.31 mm (4.264") which is a little over 4.250" "
Seems like there might be plenty of clearance in the form of backlash. I guess it is not important when adjusting the tilt of the burner that this mechanism controls.
Or we are counting on pulverized coal drifting around and causing problems at adjustment time. Or when made as dimensioned with the proper C-C mounting distance it would bind, so the solution just before the Great Depression was to mount the shafts a little further apart. Or, some other situation altogether.