Gearbox

Last Updated: 3 December 2010 - added commentary about gearbox oil flowing out

Disassembly

Before removing the gearbox the reversing geartrain must be partially removed. Specifically, remove the idler gear, 48-tooth stud gear, screw gear, and banjo casting (in that order).

The nut holding the idler gear in place is captive to a square-head screw in the banjo casting that prevents it from turning with the gear.

The nut holding the screw gear, however, is attached to the gearbox input shaft and will rotate with it. This made it tricky to remove without a back gear in place to lock the spindle down. The nut isn't tight, but too tight to hold the gear by hand while unscrewing it. My solution, shown in the lower photo, was to clamp a pair of Vise Grips to the screw gear. I used a light pressure on the pliers, and the jaws are contacting the gear only on the front and rear faces, in an area that is not used as a bearing or sliding surface and has no impact on the function of the screw gear. Clamping pressure was so light that the jaws left no discernable marks on the gear. If your back gears are installed, you can achieve the same result by engaging back gear and leaving the bull pin locked. This will lock the spindle, which will also lock the gearbox as long as the reversing geartrain isn't in neutral. The only advantage to my method is it prevents any possibility of chipping a gear tooth if undue torque is required to remove the nut.

There are 2 stud gears for this lathe - a 24 tooth and a 48 tooth. For all threads above 7 TPI, the 24 tooth gear is used. In these photos, notice the 24 tooth gear is installed in the stud gear position while the 48 tooth gear is merely "stored" in position on the end of the gearbox input shaft. This is the way it came from the factory.

Here's what the box looks like with all the necessary gears removed. All that remains in this photo is to loosen the allen head screw that clamps the banjo fitting to the gearbox casting. I found even with the screw removed it's a tight fit, so pull hard and rotate.

To remove the box from the bed there were 4 screws on my machine. It's possible there are more on later-model lathes, so be careful. The first photo here shows the top 3 screws, all slotted-head, that penetrate through the lathe bed right at the front vee-way. These screws were not abnormally tight, and I was able to remove them with an ordinary screwdriver.

The fourth screw was a hex-head cap screw, located perpendicular to those shown in the first photo, down in the lower right corner of the gearbox. [My intention is to post a photo showing the location of this screw, but the only one I took at the time of disassembly is terribly out of focus. Look for a proper photo after I get things back together].

The best thing to do is remove the bottom-right-corner screw first, followed by the top 3 screws. The gearbox is somewhat heavy (maybe 25 lb without the leadscrew), so gravity will want to pull it away from the bed. Be sure it's supported when the last screw is removed! Don't let it hang from the end of the leadscrew or almost certain catastrophe will result!

The second photo here is the complete gearbox and leadscrew assembly as I removed them and placed them on the bench. I now believe it's possible to remove (and install) the gearbox alone, but for the first time you remove one of these you'll want to have 2 people present. Above all, you want to avoid bending the leadscrew. And remember you'll need double the length of your lathe in clearance on the left end to have room to pull out the entire leadscrew!

First, rotate the leadscrew until the keyway is pointed straight down. This is done so that the key in the apron worm drive assembly doesn't drop out of it's slot when the leadscrew is pulled out.

Move the carriage so it's roughly at the center of the bed. Have your assistant stand at the tailstock-end of the leadscrew. You stand at the gearbox.

First loosen all four screws. Then remove the hex-head screw (the one at the bottom right corner of the gearbox). Now you want to support the load of the gearbox with one hand while you unscrew each of the top screws, working from right-to-left. When you get to the last screw, you'll be bearing the entire weight of the gearbox with your hand.

Once the screws are removed, begin sliding the leadscrew out of the apron to the left. Your assistant should be supporting and guiding the far right end of the leadscrew. Once the right end disappears into the apron, have him support it at the left end of the apron until the end re-emerges. Bring the entire assembly somewhere convenient at set it down as shown in the last photo, with a block to support the end of the leadscrew.

You'll need to do most of the work with the box upside down, so it might be a good idea to turn it over while your assistant is still present.

The third photo here shows a closeup of the lower-right corner of the gearbox with the "parellelism adjustor" (my terminology). It's a hollow hex-head screw that butts up against the lathe bed to define the distance between the gearbox and the bed. By adjusting it in or out you can adjust the alignment of the gearbox parallel with the bed. Only minor adjustment is possible, since there's only a little "slop" in the 3 screws that secure the box at the bed. I was careful not to move the position of that adjustor, and thereby avoid having to adjust the gearbox position later.

And here's a view of the gearbox from the back, freshly removed from the lathe bed. I have labeled the major portions with my own terminology.

You'll want 2 people present when you remove the gearbox because the leadscrew is attached and you don't want to bend it.

This is a simple 40-speed constant-mesh gearbox with 4 shafts: input, shift rail, mainshaft, and output. The design is robust, easily serviced, and strong. My only complaint is the lack of an oil sump. It uses a wick lubrication scheme. The red arrows in the photo are an attempt to indicate the power flow through the gearbox.

1. Power enters through the input shaft.
2. The input shaft transfers power over to the left side mainshaft gears, which rotate independently of the mainshaft.
3. The left side mainshaft gears transfer power into the left side gear shifter, which contains 2 gears.
4. The left side gear shifter transfers power to the shift rail, where it's picked up by the right side gear shifter.
5. The right side gear shifter transfers power to the right side mainshaft gears, which are keyed to rotate with the mainshaft.
6. The right side mainshaft gear transfers power to the output shaft.
7. The leadscrew is attached to the output shaft.

As you can see, gear changes are affected by sliding the two shift levers along the shift rail, thereby engaging different gears on the left and right side mainshaft gearsets. In my opinion, this design should never be shifted during rotation.

Begin dismantling by removing the jamb nut and retaining nut for the leadscrew. If you look carefully in the previous photo on this page, you'll see I've already loosened the jamb nut (lower right corner of the photo). You'll need 2 big (>1") open-end wrenches to grab these nuts. I used 2 Crescent wrenches. The jamb nut isn't particularly tight, and the inner nut sets a sort of bearing preload, so it's not very tight either. In fact, mine was little more than finger tight.

With the nuts removed, the gear slides off when you rotate the mainshaft so the gear teeth line up to just the right position.

The leadscrew is NOT pressed into either it's inner or outer bearing. But if you've got it sitting horizontally there's a good chance it'll bind in the bearings just enough to prevent removal. I found it's best to swing the gearbox 90° so that the leadscrew is pointed straight up. This relieves any binding and allows the shaft to slide out.

The middle photo shows what you're left with in the case. The inner leadscrew bearing is lightly pressed into the casting as shown. To remove it, carefully punch in (toward the camera) using a pin punch on the inner race.

Notice also there is a tiny felt wick inside the bore where the output shaft mounts. It must be gently pried out with a small pin of some sort.

Next to be removed is the shift rail (perhaps more accurately called the countershaft). To do so, use a punch to drive out the retaining pin as shown. The pin engages a groove in the shift rail. Once the pin is removed, drive the shift rail out as shown in the second photo. It must be driven this way - attempting to remove it from the other direction won't work, since each end of the shaft is a different diameter.

Note: there are felt wicks in the keyway at the smaller-diameter end of this shaft. At the large diameter end, there is one wick in the gearbox casting itself, as shown in the third photo at right.

The next step is to remove the mainshaft, which is where most of the gears are mounted. It is held in place by a single taper pin, located toward the edge of the smallest gear, as shown in the first photo. This is a tricky location for a taper pin, because you're near the center of the mainshaft and you don't want to risk bending it by wailing on the pin.

I pounded on the pin until I was no longer comfortable with the amount of force required to remove it. So I decided to drill out the pin instead. There are pitfalls to doing this, and I fell right into one. When I drilled I was off center slightly, which placed the bit on a path at a slight angle to the actual pin. So I ended up with an enlarged hole at the bit entrance and almost 2 separate holes where the bit exited (the original hole and the hole made by the drill bit offset from the original by almost 1 drill diameter). I used a number 16 drill bit.

Fortunately, I was able to save some bits of the original pin, so I placed them back into position in the shaft. The pin hole diameter is around 0.165", so I filed down a piece of 0.1875" stainless rod until it fit "snug" in the now-enlarged hole. I believe the repair will hold fine, but learn from my mistake: if you need to drill out this pin, be careful your bit is precisely aligned with the centerline of the mainshaft!

The second photo here shows the mainshaft being removed, which must be done left-to-right, since the diameter is larger on the right side of the shaft. It's not press-fit into anything, so it slides out pretty easily with some taps from a hammer. If possible, I recommend preserving the left/right orientation of each of the gears in the stackup, if possible, so that they can be reassembled the same way. Decades of use will have worn the gear teeth in a certain pattern that is probably worth preserving in the interest of strength.

With the mainshaft removed, you can remove all the main gearset. In this photo I'm showing the two stackups: on the left are the gears which are keyed to the mainshaft, on the right are gears which rotate independently of the mainshaft. Notice I've run wire ties through the center of each stack so I can keep the gears in the proper orientation.

Once the mainshaft and gears have been removed the output shaft inner bearing can be removed. I did this by carefully tapping it out from the back (the right side in the photo) at the inner race. It's not very tightly pressed into the casting, but there remains the possibility of separating the bearing inner race from the outer race by this method. If this should happen, removing this bearing may prove singularly troublesome, since there's no convenient way to access the outer race for tapping.

Fortunately, it's a standard size ball bearing, and if you must destroy the old one to remove it finding a replacement will be simple. The bearing I removed was a New Departure number 20205.

At this point I found the only major damage to this gearbox. The bearing shown here is for the inner portion of the input shaft, and it's badly scored. The input shaft is also scored, and it looks like something solid went through this bearing. It's not clear whether that's what happened, or whether there was a lubrication issue. It is interesting to note the lack of an oil wick here. However, on reassembly, I decided that any shaft with a wiper embedded in it probably doesn't need a wick in the bearing. The input shaft has a felt wiper at this location, so I didn't bother to install wick in the hole. This may or may not be correct, but at least if this bearing completely degrades it's easy to replace.

I removed the bearing from the casting - it's press fit, and I reduced the tightness of the fit by filing down the bearing OD slightly. I prefer a nice slip fit.

The bearing is a standard size - 1" OD x 3/4" ID (for a 3/4" shaft). I was able to buy an inexpensive replacement from McMaster-Carr, in case I decide to replace this damaged one. It's interesting to note that although this bearing has suffered severe damage, the clearance remains right about 0.001" and the shaft rotates very smoothly. For those reasons, I've returned the bearing to regular service and will monitor it carefully.

Here's the gearbox empty showing the main oil rifle. This passage feeds the input, output, and mainshaft bearings.

The lubrication scheme is worth taking a moment to examine. The second photo here shows the output side of the gearbox, with relevant portions labeled. The main oil rifle feeds the mainshaft and output shaft directly. Although the output shaft rides its own bearings, the passage in the case penetrating the output shaft hole provides a small flow of oil that will eventually find its way into the bearings.

Some of the oil in the mainshaft bearing is picked up by a wick, which carries it down a secondary passage to the shift rail (countershaft) bearing. Personally, I don't like this kind of serial lubrication scheme, because I think it has a tendency to under-lubricate the shift rail bearing.

The third picture here shows the input side of the gearbox labeled to show oil flow. Not visible is an oil hole at the top of the mainshaft bearing.

There are numerous wipers and wicks throughout the gear case, and I've replaced all of them. Originally, every hole had a wick in it, but I've followed a slightly different philosophy. The mainshaft and input shaft both have embedded felt wipers. My feeling is that these bearings don't need wicks in the oil passages that feed them, since oil will merely flow out from the main passage and into the shaft bearing, where it's picked up and stored by the shaft wiper.

The felt used here is from a sheet of 1/8" thick F10. The F10 works well here, because you need something that's easily compressible.

You may not agree with these changes to the lubrication scheme, and I encourge others to follow their own logic in the matter.

Operational Experience
Since I've begun operating the lathe gear box, I've found that oil poured into the main rifle runs out rather quickly. As a result, I find myself adding oil to the gearbox frequently - once before first use, then again roughly every 30 minutes of use. However, everything in the box is well-soaked with oil (and makes a nice puddle beneath it). I bought a small aluminum baking tin at the grocery store which I've stuck beneath the gearbox using a couple strong magnets to secure it to the underdrive housing. This catches all the oil that drips off, leaving the floor clean.

In addition to the main oiler port, I've also been adding oil directly to the felts in the gear case on either side. I show photos of the locations on the lubrication page (items No. 8 and 9). This makes absolutely certain that all the bearings are well-oiled.

Update - 12/3/10
I've been considering lately whether I should've reinstalled the felts in the main oil galleries rather than letting the shaft wipers hold all the oil. The reason is, the main shaft is unable to store any oil - it all runs out. Yes, the shafts are all being very well lubricated (it's clearly visible when I fill the gearbox), but it's annoying that all this oil tends to flow right out of the gearbox over a fairly short period of time.

Consider this carefully when choosing which parts of this rebuild to follow, and which to dismiss as "foolhearty".

Reassembly

The box is reassembled beginning with the mainshaft. This can be a little tricky, because you must have the entire main gearset in position before inserting the mainshaft. That's because the length of the gearset is the same as the inner width of the casting - there's no room to slide gears onto the shaft one at a time. The problem is compounded by the need to align the keyways on the right side gearset.

I marked all the gear teeth over the keyway on each gear with a Sharpie to aid in alignment. I used the input shaft as a temporary hold for the left side mainshaft gears - the input shaft OD (on the smaller end) is the same as the mainshaft OD. Insert the input shaft from outside the case into the mainshaft bearing on the left side, and into the 3 gears that make up the left side main gears. This holds them in position while you slide the right side mainshaft gear stack. When the mainshaft is driven through, it will push out the input shaft.

I understand this isn't a very good way of describing reassembly of the mainshaft. It's a tricky - but not difficult - process. The best advice I can give is to try it a few different ways and come up with your own method.

Before any of that happens, however, you must replace the felt wiper. The first photo here shows the new felt I installed. I experimented with a number of different options, and decided that 1/8"-thick, 1/2"-wide F1 felt works best. To make the wiper, cut a strip of the F1 to the proper length. Notice I left about 1/2" sticking off the end of the shaft. I did this because I figure I can add a drop of oil to it before each use to help augment the factory lubrication system.

The F1 is almost incompressible, and the groove for the wiper is slightly less than 1/8" (it measures at about 0.120"), so I had to carefully shove the felt into the groove. Once it's in there, use a sharp razor blade to trim the felt flush with the surface of the shaft. This may take some practice - I had to make several before I was happy with the technique. Since F1 is basically incompressible, it's important that it not be too proud of the shaft surface. You want it to stick out just slightly so it gently contacts the inside of the case bearing surface.

I did try using F10 here, but installing the shaft can be a rough process. When I inserted the shaft in the gears, the sharp edges caught on the F10 and tore it out. F1 is far more resilient, and there were no problems getting the shaft through the gears.

The second photo shows the mainshaft finished.

The input shaft is installed next. I've noted the inner bearing here (the one that is severely damaged on my gearbox), as well as the collar used to retain the input shaft. This goes together much easier than the mainshaft.

I generally shy away from press fit parts, and tend to file any such parts down to a slip fit. I've done so here with the inner bearing. Originally it required a substantial beating with a hammer to remove the bearing (so much so that I actually deformed the bearing trying to remove it). So I actually chucked it up in the lathe (since the headstock and electrical work was finished) and filed down the OD a few thousandths. Now it can be pressed in by hand, and is retained securely by the original retention pin (not visible in this photo).

Next is the shift rail (or perhaps countershaft, depending on how you prefer to name your gearbox shafts). This was the easiest item to reinstall, because the shifters can be positioned to hold themselves pretty close to the correct position.

In the first photo I'm showing my method for putting new felts in the keyway. That's a piece of 1/8"-thick F1 trimmed narrow enough to be forced into the groove. In the photo I'm using a fresh razor blade to trim away the protruding felt so it's flush with the surface of the shaft. As I've noted in the photo, be sure to press down firmly with your finger the area just ahead of the blade. If you don't do this along the whole length as you cut, the blade has a tendency to pull the felt up out of the groove as it cuts, which leaves the felt too far below the surface of the shaft to do any good.

In the lower photo the gearbox is fully reassembled except for the leadscrew, which won't be reinstalled until the box is ready to go back on the bed. Remember how dirty this was?

The 2 gears that slide along the shift rail have 2 keys in them, corresponding to the 2 keyways in the shift rail (only one keyway is visible in this photo). The felts for the shift rail mount only on the left end, and I've noted one sticking out slightly in the photo. I cut these felts slightly too long so they can be manually lubricated and their condition judged without dismantling the box.

Lubricant
South Bend specifies a gearbox oil of type "B", which has a saybolt universal viscosity rating of 150-240 seconds at 100°F. According to Machinery's Handbook, the conversion from saybolt viscosity to centistokes is:

cst @ 40°C = 0.22*saybolt-180/saybolt

which suggests an oil between 31.8 cst and 52.05 cst would be appropriate. It turns out the manual transmission fluid I use for my truck, Pennzoil Synchromesh, has a viscosity of 41.6 cst. I trust this oil in my truck transmission, and have no reason not to trust it here.

I left the gearbox off the lathe and separated from the leadscrew for several months while I readied other parts of the lathe. I stored the leadscrew vertically (or very nearly so) in order to prevent putting any kind of curve in it.

When it comes time to reinstall the gearbox, you must attach the leadscrew before mounting the gearbox back on the lathe. This is because there's not enough clearance to get wrenches under the gearbox for attaching the leadscrew nuts.

As I mentioned earlier on this page there are two nuts that secure the leadscrew. One sets the preload on the inner and outer bearings, and the other is merely a jam nut. You don't need much torque on the inner nut! There's no need to put a lot of preload on those bearings. The way I did it was to tighten the inner nut until I just couldn't rotate the outer race of the outer bearing by hand. Unscientific, at best, but I think this should work.

Getting the leadscrew properly installed in the gearbox while sitting on a workbench isn't trivial. It took me awhile to get it installed and the nuts tightened with the leadscrew in proper alignment to the case. Take your time, and try to minimize any torque you're putting on the gearbox with the leadscrew.

My method was to stack some scrap wood beneath the far end of the leadscrew so that when it sat horizontally it was at nearly the correct height for the gearbox hole. The photo here is showing that I had the gearbox positioned upside down, since that places the leadscrew closer to the bench surface. Not to mention it's easiest to get wrenches on the nuts with the gearbox in this position.

You'll want somewhere around 1-inch of wood stacked at the far end of the leadscrew to minimize the binding where it enters the gearbox. My method was:

1. Stack a 1" block at the far end of the leadscrew.
2. Slide the leadscrew into the gearbox and install the output gear and the two nuts (finger tight).
3. Torque the inner nut until you can't move the outer bearing race by hand anymore.
4. Rotate the gearbox assembly so it's right-side-up, being careful not to put too much stress on the leadscrew.
5. Stack a block several inches high at the far end of the leadscrew. Use enough height to take all the stress off the leadscrew where it enters the gearbox, so that the whole thing is balanced.
6. Check the tension on the leadscrew outer bearing by trying to rotate the outer bearing race by hand.
7. Use a wrench to make minor adjustments to the inner nut as necessary so the bearings are just tight enough so the outer bearing race can't be rotated by hand.

I went through this procedure several times until I was happy.

When you're ready to reinstall the gearbox and leadscrew, you'll need an assistant to hold and steady the far end of the leadscrew. If the apron is installed, rotate the leadscrew so the keyway is pointing down, and rotate the worm drive in the apron so the key is on the bottom. Position the apron about 1/3 of the way down the bed from the headstock-end. This will put it in a good position to provide support to the leadscrew close to the gearbox during the install.

Be careful to never let the gearbox hang from the end of the leadscrew! Slide the leadscrew into position and screw it down before your assistant leaves. Once the leadscrew is installed it's best to have the assistant help you hold up the gearbox (it's heavy), since the far end of the leadscrew is now supported in its bearing.