Worm Gear Gearbox

[Matt Adams]

Originally Posted by dddriveman

I would love to take a look at that worm tranny. If you do not mind sending it to me. You can find my email in my profile.

I don't want to jump in and be a nay-sayer... but I think one of the things you need to really weigh in before considering a design using a worm gear is the immense efficiency loss you're going to have simple due to the nature of the gear set.

We're talking in the range of efficiencies as low at 30% !

The advantage you mentioned, about not being able to back-drive the gear set, is not always true. There is some critical pressure angle and coefficient of friction that leads to the point were you actually can back-drive this gear set. Jim said that it's above a reduction of 50-1. I can't verify this either way.
[edit: But Paul definitely can. Rock on!]

However, simply because of the efficiency issue, I would steer clear of them in a drive-train gearbox unless there's a really good engineering justification. For a large reduction in a small amount of space, an off-the-shelf planetary gear set would probably hold better efficiencies.

That's my two cents. Good luck

Matt

[Paul Copioli]

Back Drive-ability is a function of surface finish, lubricity, and worm lead angle. Using a bronze worm gear and a hardened steel worm (the small screw looking one), the max lead angle is 5 degrees to insure self locking. This is not a guarantee! What is a guarantee is that above 5 degrees, the worm gear set will not lock. The lead angle is the reason the window motor locks and the van door motor does not lock. It really has nothing to do with the gear ratio. A gear ratio of 50:1 with a lead angle of 15 degrees will not lock.

However, the more thread starts on the worm, the higher the lead angle and the lower the gear ratio is. Remember, gear ratio for a worm gear is # of teeth on the gear divided by number of thread starts on the worm.

-Paul

[greencactus3]

a little off, but for lego, wormgear 24:1 reductions work very well, for lego robots i mean. not too slow, yet (well since lego isnt efficient whatsoever,) efficient (make sure you wupport both axles from both directions, and since lego axles are plastic, support them as close to the gears as possible.
well, yea. worm gear gear boxes (wormgearboxes?) as an idea are very applicable ithink. except as with designing any kind of gearbox, will have difficult parts but if it works well, great!

...and yea, im understanding why my english teachers rarely give me any good grades on my essays.

[Adam Y.]

Originally Posted by Matt Adams

I don't want to jump in and be a nay-sayer... but I think one of the things you need to really weigh in before considering a design using a worm gear is the immense efficiency loss you're going to have simple due to the nature of the gear set.

We're talking in the range of efficiencies as low at 30% !

Actually efficiency might not be an issue if you properly design the gearbox. A little bit of engineering and you should be able to get the efficiency fairly high. Of course a little research shows that the efficiency can reach 60%.
Article
Ps. Please someone build a drivetrain using these gears. These are the funniest sounding words ever. Hypoid and globoid.

[Cory]

Originally Posted by Adam Y.

Actually efficiency might not be an issue if you properly design the gearbox. A little bit of engineering and you should be able to get the efficiency fairly high. Of course a little research shows that the efficiency can reach 60%.
Article
Ps. Please someone build a drivetrain using these gears. These are the funniest sounding words ever. Hypoid and globoid.

60% efficiency is still downright terrible for a drivetrain when you consider spur gears are 98% efficient and chain is 95% efficient

[Adam Y.]

60% efficiency is still downright terrible for a drivetrain when you consider spur gears are 98% efficient and chain is 95% efficient

Read the article. It's actually possible to reach that degree of efficiency using regular worm gears.

[Paul Copioli]

Here is a picture of a hypoid gear. It is used on an automotive painting robot that my company manufactures.

[Paul Copioli]

Here are some facts about worm gear efficiency:


Efficiency = Tan(Lead)*(1-f*Tan(Lead))/(f + Tan(Lead))

Lead = Worm Lead Angle
f = Coefficient of Friction (Mu)
For a bronze gear and a steel worm f = 0.05
Easy to purchase worm gears have lead angles ranging from 4.5 degrees to 22 degrees. Here is the bounding range on efficiencies

Lead angle of 4.5 degrees has an efficiency of about 61%

Lead angle of 22 degrees has an efficiency of about 87%

This assumes everything is lined up!! Tolerances are very critical for a worm gear set-up.

The lead angle of 22 degrees corresponds to worms with 4 thread starts and the lead angle of 4.5 corresponds to worms with 1 thread start. The worm with one thread start has 4 times the gear ratio for the same package size than the worm with 4 starts. A worm gear box has several advantages for FIRST robots. The main two are package size and weight. We have used worm gears on our robots for three years, but only once on our drive transmission (see white papers under CCT).

[Matt Adams]

Originally Posted by Adam Y.

Actually efficiency might not be an issue if you properly design the gearbox. A little bit of engineering and you should be able to get the efficiency fairly high. Of course a little research shows that the efficiency can reach 60%.

I'm going to have to strongly disagree about efficiency not being an issue.

Even with your research, which I won't refute, 40% losses in a single stage of a gear set is still quite large compared to the 10%-15% losses over an entire drive train with spur gears and chain.

It should be noted that Adam's article did cite a 97% efficiency for a multiple stage gear train using worm gears, which I found to be quite impressive. (Costs? Yikes! )

Worm gears have their place, but I would strongly discourage their use on a drive train if you're just looking to reduce your gear ratio. I'd definitely like to see justification addresing the losses, which we can all admit are several times larger than spur gears.

(See Paul's link for an appropriate application.)

Matt

[Peter Matteson]

The worms we used last year were 35:1 on the back of another transmission. I don't recall the ratio for the Bosch tranny we used however. We wanted an extremely high torque setup for our application though.

Pete

[Jim Schaddelee]

Here is a design for positioning worm transmission from team 107 , the nice thing about our design is that there is 50 to 1 reduction on the worm which should make it not back drive. yet you can quickly change speeds and motor combination. It also can have either a pot or encoder mounted simply.



Thanks Jim schaddelee team 107

[Doug G]

We used two FP's last year to build a winch for climbing the bar. Be very careful about efficiency loss - we had ~ 30% loss. But it did lock up nicely and keep us hanging

Here's an mpeg of it running

[Jim Schaddelee]

Doug

That looks great ,what was your reduction and did you lubricate the worm with anything?

Jim Schaddelee team 107

[Adam Y.]

Worm gears have their place, but I would strongly discourage their use on a drive train if you're just looking to reduce your gear ratio. I'd definitely like to see justification addresing the losses, which we can all admit are several times larger than spur gears.

To be honest I have seen a design that could work but it is really weird. It's the triple crown of complexity because it combines a worm gear, a cone drive, and a differential to increase the horsepower of a motor. The only disadvantage is speed is lost.