View Full Version : How does a quick change gear box work?

tony ennis
01-16-2009, 01:50 PM
I don't have one, obviously. My pal does, but he thinks it's more important to teach his son math than to talk to me about this. What a loser. :p

I've seen pictures of the insides but nothing that explains how they work. The part I find mysterious is how gears of different diameters - based upon the handle settings - are made to connect to fixed shafts (leadscrew and spindle.)

If someone has a short video, that would be greeatttttt. yeeeahh.

01-16-2009, 02:04 PM
That's a tall order ......

I dont know how anyone here, can explain that away without dozens of pictures and diagrams ..and make themselves understould .

There are no vids, as far as i know, of any lathe quick change gearbox.

Best take the gearbox off your southbend ...(twenty mins work...probably needs a good clean anyway) ..turn it upside down ..get the lad to revolve the input shaft after you do each gear change ...
Then ,you , he and everyone will have it all worked out in mins .
all the best.markj

01-16-2009, 02:18 PM
If you know a pillar drill v belt system works i.e. 2 cones of different size pulleys stacked together on shafts one the other way to the other with a belt linking them? Then you can easily understand the gear box

Imagine instead of pulleys there are gears. There are two "cones" of stacked gears one the other way up and parallel to the other and are not in direct contact.

Now we have another small gear (an "idler") held on a swingable ,slidlable shaft to connect the gears opposite each other.

Thus the idler serves the same function as the belt in pillar drill v-belt system in that it selects the ratio to be used.

01-16-2009, 02:33 PM
Inverse stacks are not necessary or even typical to my knowledge-One stack on the input shaft, tumbler arm with idler gear travels along the stack, picks off proper gear and transmits that motion to the output shaft. (lead screw/feed shaft)

On Edit: The tumbler gear is in mesh with a gear splined onto the output shaft.

A Hi/Lo range selector or three ahead of the quick change multiply available ratios.

Some lathes (Clausing 1500 for instance) use planetary gear sets for the quick change box.

tony ennis
01-16-2009, 02:41 PM
tumbler arm with idler gear travels along the stack, picks off proper gear and transmits that motion to the output shaft.

That's the mystery to me. Now when you say "picks off" I suppose that means engages. But each of the gears on the stack has a different diameter. Does the lever (tumbler arm?) just jam the idler into both gears any which way it will fit and the spring-loaded lever keeps it in place?

01-16-2009, 02:42 PM
Inverse gear stacks are the most common on hobby/homeshop lathes. They are, to my limited knowledge typified by the spring loaded pin/hole arrangement. As Derek posted, the power is transferred by an idler gear held in mesh with the appropriate pair of gears in the inverse stacks by the link held in positon by the spring loaded pin. The variation in gear diameter is why the holes run in a slanted line.

FWIW, as I recall from gear class, there are 3 ratios possible for each planetary set, direct ( where the set is locked), mid range, (where the carrier of the planetary gears is linked to the output), and deep reduction (where the carrier is fixed and the planetary gears act as idlers and the outer ring gear drives the out put). In an automotive auto tranny, the selection is done with hydraulic clutches. In a change gear drive, the selection is likely done with a detented position lock (knob or lever), linkage, or spring loaded pin. Interestingly enough, there are rotation reversals that are dealt with too... Seems auto transmissions need at least two planet sets to compensate for the reversals....

01-16-2009, 02:48 PM
Here's a constant mesh gearbox animation. http://www.howstuffworks.com/transmission3.htm

There's a lot of variations on this, of course. There are combination gear boxes that have constant mesh and movable idlers, sliding dogs, sprague clutches, you name it, it's been done.

My personal favorite is the ball lock shifter as used by the Hodaka motorcycle: http://www.dansmc.com/gearshifter2.htm

01-16-2009, 02:56 PM
Tony, I was editing while you were typing-may be clearer now??

Look at the "banjo" arm on the end of your lathe. It swings to allow contact with any diameter of gear. Add the output shaft as the banjo arms pivot point, a splined gear on the output shaft so the tumbler can travel along the cone stack, and a simple or compound connection between the output gear and tumbler gear and you understand the "Norton" quick change.

The detents hold the tumbler gear to the stack gear, but I think geometry and rotation direction actually counteract most of the load in some designs.

01-16-2009, 06:59 PM
Here is a pic showing the box on one of my lathes with the red line showing the power flow..


Spin Doctor
01-16-2009, 07:32 PM
Think of the selector handle as the banjo on a lathe with out a QCGB. In the gearbox you have the gears that represent the different ratios for the lead screw. In the 1-1 ratio where tha leadscrew and spindle are turning at the same speed the sliding gear on the input shaft will engage a gear with the same number of teeth (there will also be an idler gear on the selector arm, the idler can be any number of teeth). Say your leadscrew is 8TPI. Then to cut say 10TPI (or any of its muliples) then the sliding gear and the gear for that selcetor position have to be equal to the ratio of 8/10 or .8. So if the sliding gear is 24T then the one for that position would have to be 30T. The different mutiples like 20TPI or 40TPI would be accompliesd by means of a seperate set of gearing that would increase or decrease the ratio between the leadscrew and spindle.

On my lathe the leadscrew is 7TPI. I have thought about building a QCGB for the lathe at some time in the future (don't ask why I didn't buy a lathe with a QCGB, I have my reasons*). The gear train for threads would be

7TPI, 28T/28T
8TPI, 28T/32T
9TPI, 28T/36T
10TPI, 28T/40T
11TPI, 28T/44T
12TPI, 28T/48T
and so on. These ratios are predicated on there being mutiples of the direct ratios between 7TPI and the other various pitches mutilpied by 4 to get gear wheels of reasonable numbers of teeth. IE 7/8=.875, 28/32=.875. Basically it ain't rocket science folks

*The reason being I want to build a single tooth dog clutch system for the leadscrew and the best place for one is usually where the is mounted on most lathe. In this situation it would need to be mounted of the rear end of the headsctock

01-16-2009, 09:25 PM
I will borrow Ringer's pic (thanks - good pic Ringer):


The input from the change gears (which are driven by the lathe spindle) enters at the top left, down through the change-gears (left) and onto the lay-shaft (bottom), via the large gear on the lay-shaft and via the idler gear - with with it is in permanent mesh (both the large gear and idler are mounted on the selection-lever (to the right of the rising red line).

The selection lever moves left-right to position the large and idler gears opposite/in line with the required gear in the cone/stack. The selection gear is lifted so that the idler gear engages with the selected gear at the correct centre distance from the selected gear in the cone/stack. At that point, the spring-loaded plunder on the hand selection lever engages the correct hole in case/casting of the quick change selection/selector gear-box.

The cone/stack gears are keyed/splined to the output shaft and then to the lathe lead-screw and feed-shaft.

That may be hard to follow but its a bloody side harder to type/write and would not even have been attempted without Ringer's pic.

Thanks again Ringer - appreciated.

tony ennis
01-25-2009, 03:38 AM
Thanks for the diagrams. Very helpful.

For the craftsman QCGB, there's some gear fu in the left hand side - it has to multiply by 1, 2, 4, 8, 16, and 32.

01-25-2009, 03:09 PM
My personal favorite is the ball lock shifter as used by the Hodaka motorcycle: http://www.dansmc.com/gearshifter2.htm

Great links, The Honda gears are very simple and very clever.