Gear Pressure Angle and Pitch.
 

In the continuing quest to build a gearbox, yet another question has popped up. (perhaps these threads will turn into a gear FAQ?)

I've been looking around for people who sells gears. Mainly McMaster and BERG. What I've seen in the Mcmaster catalog however, is that their selection of gears of 20 degree pressure angle seem somewhat limited. Now their website states that the 20 degree pressure angle is a new AGMA standard for higher torque carrying capabilities, but they don't seem to offer any plain steel 20 degree spur gears under 24 pitch.

BERG on the other hand, has a much greater selection of gears.

My Question, mostly out of curiousity and to see other people experiences, is

-How significant is the difference between the older 14.5 degree pressure angle gears and the 20 degree gears.

-What pitch gears have you used, and how were the results?

The original plan was to use 20 pitch gears at 20 degree pressure angle, but I am having problems finding one with a finished keyed bore. (our machining resources are limited)

I have however seen that Mcmaster sells 14.5 degree pressure angle gears with finished bores, and am looking for a viability in them.

Thanks again all!

First off, I know nothing about pressure angle but I will try to answer your other questions.

Personally I would use no gears and only sprockets. They are also easy to find with finished keyway bores. Do your self a favor and don't custom make anything or weld anything. Try to find parts that will just bolt together and you'll save yourself much time and trouble.

If you insist on using gears, I actually haven't seen any stock keyway ones. We would just pin gears and use keyed sprockets. Now we don't even use gears at all and I don't want to ever again. Our experience with 32 pitch gears was to horrific to describe so just trust me and don't use 32 P. However this does present a problem if you are using the Chiaphua motor. There is nothing that will fit on or mate with that shaft other than a 32P gear such as the Bronze alloy Cluster gear supplied in the kit of parts. If the chiaphua will be used, the only good way is with a very custom CNC-ed shaft that fits over the splines on the Chia armature.


BTW - Is this whole project your presenting to us all an attempt to get a jump start on the season? If so, I think it's great. I wish my team could build a gearbox over the summer. Also, what motors will you be using and what are your thoughts on multiple gears/speeds.

Re: Gear Pressure Angle and Pitch.
 

So many questions in a single post! But I'll try to get around to them all. First, for off-the-shelf variety it is hard to beat BOSTON gear. They make all of the gears sold by SPI and many, many more. You also might want to check out http://www.globalspec.com You can spec your gear here and it will search a bunch of on-line catalogs (including BERG, but I don't think Boston) and identify any close matches. One caution, there are several custom gear houses that will tell you they have ANY gear, when they really make it up special, just for you. They will invariably show up on the match list. You can specify whether or not to have keyways on your gear.

For our purposes I don't think pressure angles matter much, as long as they match. A 25 deg gear with a 14deg gear just won't work too well.

What pitch works for a given load is very material driven. You can get away with a lot in steel that you can't in bronze or plastic. On the other hand, if you can't get steel, you might be able to reduce the pitch of a bronze gear and do it that way.

We have not yet built our own gear box, but it's definitely in the works for next year. A few years ago we built a gear driven arm though. I think the gears were close to 20 pitch cast iron, but I'd have to look it up at home. They were certainly in the SPI catalog as that's all we were allowed to use back then. They had a pretty major load, but we never failed a gear tooth. We broke plenty of other things, but not gears.

One thing I found out from working with gear design formulas. Used straight they tend to way over design things. That's because they are set up to do things that will run for many years and millions of cycles.

Could you please describe a little more? Your experience may be the basis for learning for all of us. We might be able to help you figure out what went wrong and cure your gear phobia. It also may help the rest of us not make the same mistake.

Any good technical organization is constantly doing research and development to improve it's products. Most, if not all, of the really good teams run year round to some extent or another. Even in the "off season" I'm here, monitoring the forums and seeing what ideas I can "borrow" and further develop, on the chance they will be useful next year. I also am working on a gear box design and an arm design. And yes, the gear box will shift, and will probably incorporate at least two motors.


Since we're talking about gears... Has any of you EVER broken tooth or gear inside a bosch drill motor tranny? If so I'd like to read about it!

Well, we broke multiple teeth on 32P brass spur gears we were using. They all mated fine but they just weren't suited for the job. Unfortunately brass was the only material we could get the the gears in that would fit our needs. Never broken a gear in the bosch drill tranny - remarkable. We have had major clutch slippage and have broken the green drill case and the white tranny case. We lock it in gear with a zip tie pulling the shifting lever but at high rpm it sounds like it wants to shift out of gear.

Don't Fear the Gear
 

O.K.

As I have seen in this post, several of us FIRSTers don't like gears and only swear by chain. Well, I am the complete opposite! We try to use gears everywhere; because, if designed correctly, they will last forever. Gear design entails a lot of approximations and uses a lot of derived equations, but it really boils down to 2 things:

(1) Gear Tooth Bending Stress
(2) Gear Contact Stress

For machines that need a finer precision (like NC machines and Industrial Robot Arms), we try to use a finer pitch gear (large Diametral Pitch), but because of their fine pitch, they have bending strength limitations. Contact strength is directly related to the type of material you use and the surface hardness as well as the lubrication (heat and friction are the enemy).

Gear Contact strength is the limiting factor for machines that run thousands of hours and the teeth actually wear themselves thin and has never been a source of failure for our robots.

Bending stress is also known as "We shattered our teeth into a million pieces" stress and applies to many FIRST robots. Material strength, tooth facewidth, and Diametral pitch all contribute to the tooth bending strength. Larger gear teeth hold up better(smaller Diametral Pitch, or DP), face widths up to 3/4" are good, and steel is better than brass in most instances.

I will let you in on a little secret: Brass is heavier than steel

So why would you use brass?

Steel gears from Boston Gear do come width keyways standard.

Face width is your friend, but don't get carried away. 1/2" face width for the last stages in a drivetrain are common.

Lastly, the reason the little gears (finer than 36 DP) in the drill transmission don't break is because the load is shared between 3 or 4 gears at the output. The main advantage of a planetary gear train is the ability to handle large loads in a small area.

Don't fear the gear, because it can be your best friend.

-Paul

I agree with Paul in every way (even though I don't have his mechanical background)

For our arm gearbox (with a 3.2 ft lever arm on it that was very abused last season) we used the standard little adapter gear that came with the (Edit) Chiaphua (Edit), to a brass gear that we had laying around at 32 pitch then we went to 20 pitch 20deg pressure angle for the 2 final stages and never had a problem, these gears are 'seriously strong' looks like something you would see on a small riding lawn mower. All were welded except the brass which was double dimple set-screwed and green loctited (it's never coming off)

For the drivetrain, same thing the output stages were 20 pitch 20 deg with no hint of problems on four different gearboxes.

From what I remember Dr. Joe posting straight spur gears offer about the highest efficiency of any of the different gearbox designs.

All gear gearboxes are more difficult to build than putting two sprockets on shafts and running chains between them but sized correctly they are incredibly strong.

For our metal gears, all were lightened to only 20-35% of their original weight mostly by removing most of the hub, and machining holes into the face, we didn't need a very thick hub since they were all welded

The name is Chiaphua okay.

Just curious did anyone ever think of using a worm drive system for their gearbox driving the wheels? It could pack a lot of reduction in a very small area. Also, I've seen in a white paper a gearbox using the drills motors, chiaphuas, and fisher price motors. I was wondering if any other teams use anything besides drills and chiaphuas to drive.

I had another really cool idea for a drivetrain. Have 2 motors drive a common central shaft. This shaft is input perpendicular into an axle with bevel gears driving a differential, much like the drive shaft and rear axle of a pickup. Have a giant servo or two control brakes on each side of the axle. When you want to go strait, no brakes. To turn left, pull the left brake. This would work somewhat like Jeep's VariLock axles in that each brake is controlled individually. There are no advantages to this kind of drivetrain that I could think of. There are actually many disadvantages: costly, time consuming to make, big, heavy etc etc etc. But it would still be really cool. Maybe a run at an award?

CCT uses Worm Gear
 

The CCT (check out the White Papers) uses a worm gear setup. It is very packaging efficient (lots of gear ratio in 1 stage), but horribly inefficient when it comes to torque transfer. Our worm stage had an efficiency of 70%. I would not recommend a worm gear stage for a drive train, but for a robot arm: absolutely! The worm gear has one really big advantage: It can be non-back driveable if designed right. The non-backdrive feature is a direct result of the inefficiency.

When the Van Door Motors were in the kit, I know some teams who used those for drive motors. You didn't need much gear reduction because they were already geared down. By the way, those use a worm gear.

-Paul

It sounds like Paul, Matt and I are all in agreement, that gears can be better than chains, but you have to be a little more careful with the design.

I especially agree with "face width is your friend" and machining the gears (and sprokets too for that matter!) to remove excess weight.

BTW Paul's treatment of epicyclic or planetary gears in the white papers section is very good. I've been using his equations to explore some strange corners of the drive world.:D

Baxter used both a worm gear and a planatary in their drivetrain this year to be as compact as possible and mount the motor perpendicular to the output shaft. From other posts, bevel gears can also be difficult to implement well since at least one of the gears will be unsupported on one side.

Sanddrag I edited my spelling of Chiaphua just for you

Here is a pic of Baxters Drive

Here is a pic of the modified 70 tooth gear that only weighs about 20% of the original. The width at the teeth stays full width then the rest is much reduced, even though it looks like swiss cheese the steel gears still have quite a bit of strength

I was just about ready to make a sprocket box too!
 

Interestingly enough, I was just about ready to make a sprocket box instead of a gearbox.

I don't know if it was a good thing or a bad thing now that I made and read this post :/

Very interesting experiences however, Thanks for all the feedback, and I will again as they say, go back to the drawing board.

Other places for gears:

Rush Gears
PIC Design

Matt,
Great photo, thanks for posting it. Where is the planetary? Also, is there a picture that shows what is driving the worm? From what I see near the top right of the picture, it looks as if a chiaphua is driving the worm directly. Also, how is that worm attatched to the shaft it's on, pin keyway?
Very cool.
Thanks.

You can just see the planet gears inside the brass ring on the left side. Now you know why they call it a Ring Gear, the teeth are on the inside. The Worm could have been turned out of the shaft it's on. It kind of looks that way in the picture. In which case there no need for a key or pin. If not I'd guess they used a key as there is no sign of a pin. There is a pin above the worm, but it looks like it is mostly there to hold in the bushing supporting the worm.

Confusion?
 

Looking at some of the Specs listed under the Boston Gear's Gear,

the 20 pitch steel 60 tooth gear's torque rating, it says it is 123 in.lbs @ 1200 max RPM.

Could someone explain how a gear like this would work (intuitively and through many people's experiences it should) with one drill motor alone which Stalls at 225 in lbs.

In fact the Chiaphia's 32 pitch gear was more than enough to handle it's stall loads, so why do the numbers seem to deceive here? (Is the rated torque significantly less than the maximum torque available to the gear?)

thanks again all

Sanddrag,

It is tough to tell from the picture (Plus I don't know what motor they used in this particulat photo) but the motor comes straight down to the worm drive that goes through the hub over to the left and drives a center gear, three brass gears 120 degrees apart are connected to the wheel and the outside brass gear is stationary.

Here is another pic from a slightly different angle, someone from Baxter could probably fill you in but I was impressed enough to take some pictures (Which goes for all of the pictures I post, I have the pictures because I really liked the design)

Baxter drive system.
 

I clearly see a Chiaphua armature fitting itno the internally splined shaft that the worm is on. The worm turns the worm gear and from there I'm lost. Does the large worm gear directly drive the wheel or does it drive the planerty. I see the planetary system but I have no idea what it's for or what it's driving.:confused:

Re: Baxter drive system.
 

Let me try and rephrase Matt's comment. The worm drives the worm gear. The worm gear is on a common shaft with the sun or center gear of the planetary system. The planet gears are fixed to the wheel. The outer ring gear is fixed to the frame or gear box and does not rotate. The planet gears are meshed with both the sun gear and the ring gear. As the sun gear turns, the planet gears both rotate and move forwards (or backwards, depending on your definition). Since the planet gear shafts are solidly mounted to the wheel, it too must move forward at the same rate as the planet gears. What makes this confusing is that the wheel is mounted on the same shaft as the sum and worm gears, but is not connected to it. The shaft just acts as an axel in this case. A very compact system, but it can be kind of confusing if you're trying to figure it out.

Thanks for the help. I completely understand now. Very cool.
Now that that's done with, let's get back to the origional question in this thread. Sorry if I got the thread a little sidetracked but you have to admit that it's a cool design and valuable learning about it.