Beyond Unboxing: A Dive into the Gear Vendors Overdrive

We’re going to skip ahead a little from the Summer of Ven into more the “Autumn of Ven” because I get to ramble about a cool mechanical thing. As one of the categories on this site, I often take apart neat things and make snide comments about their construction or internal parts. This time, it will be the Gear Vendors overdrive unit that lives under Vantruck, which I got for it in 2018 in a already very well used state.

I didn’t know how many miles it had on it already, and kept up with the recommended maintenance interval for fluid changes. It was just fine from 2018 until this past summer when it began slipping under load, and even worse, would occasionally seize in the overdrive state, leaving me barely able to reverse (and you’ll see why soon). Hauling so many vans back home probably did the unit in, finally.

I tried a few hacks, like flushing the fluid with a lower viscosity oil for a little while and cleaning the high pressure filter, neither of which really resolved anything decisively. Well, that’s how things shake out I suppose, so I called Gear Vendors to send the unit in for rebuild. What they do is go ahead and send you the new one and refund a core charge once they get the used unit. The total cost afterwards is about $1200 for a rebuilt unit, $500 of which is the core charge.

I’ll get to the install itself – first is the interesting bit, which is taking the old unit apart!

Here it is on the surgery ward floor after dismounting from vantruck. Notice that the input shaft is pulled out? It’s not actually attached by anything, and all you need to do is ensure it mates with the internal spline on the right side. I’ve pulled some kind of inspection lid off already, as my MO with anything on Beyond Unboxing is to start popping bolts off.

Inside the lid is the tail end of the output shaft. There’s nothing much going on here to speak of. I think this port is to install retaining clips.

The real bolts are around the perimeter on M8 studs. They come off fairly easily.

On the front, you need to remove the four nuts that holds the shifter piston’s end effectors, little bracket things, to the internal shifter ring. They’re gently spring loaded in this position, so the studs will pop inwards as you take the M8 nuts out.

When the shifter solenoid valve is enabled, oil pressure fills the two shifter cylinders (the round things behind the brackets). These two brackets then pop outward and collide some parts together inside that they’re linked to. We know from cutaway diagrams that it’s cone-clutch based, so presumably it’s connected to the cones.

Where does that oil pressure come from? The shaft that fell out has an eccentric lobe on one end, which rides in the circular socket seen in the center. That’s connected to a simple rod pump. So when you move, there is oil pressure. When you don’t move, you pray the seals are still adequate (though it seems like one or two pumps of the rod is enough to fill up the shifter piston chambers. Typically, to prevent leak-down from causing inadequate pressure to operate the pistons, the automatic controller you can get (which I didn’t install, because fuck using phone jacks as connectors) has a speed-based lockout.

And with those bolts removed, the whole thing just kind of splits open. Well there’s my first sign of something being wrong, the clutch cone has no… clutch on it.

Here are all the fun bits laid out in order left to right: the casing with the fixed clutch ring, the cone clutch assembly, the gearset, and the output. So what in the epistemological hell is going on inside these things, anyway? They rely (as most good things do) on the magic of planetary gears and their ability to add, subtract, and multiply speeds.

So the big showrunner of this basket of skulduggery is the cone clutch assembly, pictured center left. It’s coated on the outside (usually……..) and inside with clutch lining, and is the object being rammed into everything else.

I pushed the shaft back in for the assembly photo, but notice that the shaft doesn’t engage with the cone clutch, but actually with a splined bore inside the planetary carrier (center-right). When you spin the input shaft of an assembled Gear Vendor overdrive that’s at rest, you are applying torque to this carrier.

The sun gear of the planetary gearset is attached to the big clutch cone. This means the clutch cone controls whether the sun gear is fixed (grounded) or free to spin with everyone else including the clutch cone itself. This is important, because on the very right is the output cup, which acts as the ring gear (asteroid belt?) of the planetary gearset. It, too, is a cone shape on the outside, fitting tightly into the interior of the cone clutch under rest conditions.

So you have two choices for the position of the clutch cone. Either it’s spring-loaded (with a big throwout bearing seen below) into the output cup, or it’s pushed against the fixed clutch ring on the casing by the shifter solenoids.

In the rest condition, the cone clutch is shoved against the output cup. They rotate as one. With the sun gear affixed to the cone clutch, it’s forcing the planetary gears to also not rotate. If you overtorque the system and overcome the cone clutch friction, there’s one more line of defense left: the planetary carrier on one side is a very large spline that fits into the corresponding spline in the output cup, around which is a one-way bearing.

Therefore, in the rest condition with the vehicle powering forward, the torque is taken up by either the cone clutch friction or the one-way bearing forcing the carrier’s torque (which is your input, remember) straight to the output shaft.

In the overdrive condition, the cone clutch shifts forward a half inch or so and mates with the fixed clutch ring on the outer casing. The sun gear is stopped dead, and the planetary gears are forced to mate in orbit around it.

The good trick here is when you have a planetary gearset in that condition, the rotational speed of the planets adds to the rotational speed of the carrier. A gear that mates with the planets, like the output cup, will get flung faster than the rate at which you spin the carrier (/the input shaft). You might say it’s overdriven. In this situation, the one-way bearing capitulates and spins freely as the output cup is now moving positively (faster, in the same direction) relative to the carrier.

So there you go. That’s how the thing works in forward. Here’s a better view of the stackup from the end view, showing the big throwout bearing that controls the cone clutch and the also-conical shape of the output cup, and its one-way bearing inside.

When using a GV unit, you’re advised to not reverse while the unit is active (shifted in OD). The automatic controller (which I didn’t install, because fuck using phone jacks for anything) has a reversing switch lockout so it automatically disengages. Also, if a unit is severely worn, reverse is also generally difficult or impossible as it slips even in the rest state. The reason this happens is a unique property of the angle of cut of those planetary gears (they’re helical cut) and the liability of having a one-way clutch.

Cone clutches work on the principle that the two cones crammed together create an outward normal force many times the force pushing them together axially. The same principle governs tapered spindles in machinery. To work properly though, you generally have to keep holding them together. If a force exists that pulls the cones apart, they’ll decouple quickly.

The helix angle of the planetary gears creates an axial thrust load when the gears are transmitting torque. The direction of the helix is very important here. It turns out, by observation, that the helix is purposefully set to 1. Pull the clutch cone tighter in the OD and forward-torque condition, as well as 2. failsafe in the off (rest) position.

In the overdrive position, with the input shaft applying positive torque “Forward”, the helical thrust force pushes the sun gear into the clutch cone, which forces it more into the fixed clutch ring. That is, to the left in my explosion photo. It took me a while of staring to realize this is the case, and it makes the engagement more solid: As long as you’re applying drive torque, the system will tend to keep itself engaged.

Reversing in the overdrive position is a problem, because now the thrust force is pulling the clutch cone away from the fixed clutch ring (Towards the right, in my first exploded photo). At this point, you’re fighting the oil pressure in the shift pistons. At low-to-no speed, the oil pressure is not high, and likely leaking down slowly. The applied reverse torque at standstill wins, the cone clutch begins disengaging from the fixed clutch ring, and you don’t go anywhere fast while slowly grinding the clutch down. In real life, this is manifested as a really ugly squealing sound as you only maintain roughly idle speed in reverse, if at all.

Oh yeah, that one-way bearing isn’t helping you in this case either, because if you’re spinning the planetary carrier the wrong way, it doesn’t engage.

In the rest position, however, with the clutch cone now forced against the cone of the output cup, reversing torque now turns into thrust load which shoves those two cones together. The one-way bearing still doesn’t help, but at least your system is trying to be in a positive-feedback state. The reversing torque is still ultimately limited by a 3/4″ wide clutch band, and if this band becomes too worn or disappears, you could lose reversing power entirely. However, the general story seems to be it’ll try its darndest even severely neglected going metal on metal, because of the thrust of the planetary gears helping keep the cones together.

So there we go. Quite a clever mechanism with a few foolproof design choices to improve overall reliability. If the helical gears were cut with the spiral the other way, everything would have sucked more. That’s easily something I see myself doing, so good thing I didn’t design this thing, huh?

Alright, I’ve satisfied my curiosity and am now ready to put it back together so I can ship the unit back to Gear Vendors. I dunno what it is that I did, but I can’t quite get the thing to close again.

So I just left it halfway open, put the nuts and washers in a baggie, and left a note explaining myself.

That’s all for this episode of Beyond Unboxing! The installation of the unit itself will be covered separately as the Summer of Ven progresses.