As far as I know, there have only ever been two domestic sewing machines made with a true low gear. One is the 1970′s Viking 6570, and the other is the Grasshopper. Unfortunately that low gear’s gone missing from a lot of machines over the years, so in case you’re not familiar with it, here’s what we’re talking about …
That black gizmo is properly called the Speed Reducer, the casing contains a small gear and a big gear, and fitting it does three things to a Grasshopper. It slows down the drive such that for any given position of the knee-lever, the machine sews five times slower than it normally would. It increases piercing power. And it makes the machine noisy. So, let’s have a look at the thing …
Above we see the drive end of a Grasshopper as it normally is. That shaft with the slotted end to the right of the picture goes by the name of the eccentric axle (we’ll soon see why), and its sole purpose is to be the mounting for the speed reducer.
To fit the speed reducer to a Grasshopper, you line it up so that the long gear with the hole through it fits onto that eccentric axle, and the short bush sticking out the narrow end fits over the motor shaft. You then slide the thing forward and wiggle it so the teeth of the speed reducer gear mesh with the teeth of the gear on the motor shaft, then push it just a bit futher until it clicks into place like in the picture below.
That’s the theory. In practice, you may find that you can’t get the thing on properly because however hard you try, you can’t get the gears to mesh because they need to be further apart. This is where the eccentric axle’s eccentricity comes into play …
and as you can see, the reduced diameter (which fits into the motor housing) is off-centre. Now you know why that axle has a slot in the rounded end of it. Yep, if you slacken off the locking screw with the speed reducer in place, you can then rotate the axle eccentrically and vary the distance between it and the motor shaft so as to set the correct meshing of the the two gears. OK it’s a bit of a fiddle, but it works – providing you remember to tighten the locking screw once you’ve got the gears meshing properly.
So how does the speed reducer actually work then? By sound engineering principles and ingenuity is the short answer, but here’s the long one.
First off, you need to bear in mind that the motor belt pulley and the gear on the motor shaft are one assembly. They never move relative to each other. Furthermore, all that stops them rotating freely about the motor shaft is the spring-loaded key seen in the picture below. That’s what locks them to the shaft so that the motor can drive the machine via the pulley and belt.
That key is the means by which the Grasshopper automatically disconnects the drive to the needle bar when you wind a bobbin. As you push the bobbin onto the shaft, it depresses the key, which disconnects the drive to the pulley/gear, which then can’t drive the belt, which means there’s no drive transmitted from the motor to the needle bar. Taking the bobbin off allows the key to spring back up and engage with the pulley again, thereby restoring normal operation.
Getting back to the speed reducer, here’s what’s inside the casing. Note that the small gear A and the bush which goes over the motor shaft is one piece, as is the large gear B and the long gear C which goes over the eccentric axle.
Fitting the speed reducer depresses the key and disconnects the drive to the belt pulley/gear assembly in exactly the same way as putting a bobbin on the end of the motor shaft in order to wind it does. The difference is that the bore of that bush poking out the small end of the speed reducer has a shallow groove in it, which engages with the key on the motor shaft just enough to lock itself to it while keeping the key depressed enough to allow the belt pulley/gear assembly to freewheel. The motor therefore drives the speed reducer instead of the belt pulley/gear.
So, with the speed reducer fitted as in that picture above …
The motor shaft is driving small gear A of the speed reduction unit – not the pulley/gear assembly, which is now free to rotate about the shaft.
The small gear A inside the speed reduction unit is meshed with the big gear B inside it. That big gear B and the long gear C which fits over the eccentric axle are one piece of steel.
That long gear C meshes with the gear on the motor shaft which is one piece with the belt pulley.
So the drive train is motor shaft > speed reduction small gear A > speed reduction big gear B > speed reduction long gear C > big gear/belt pulley which is freewheeeling on motor shaft, and from there to the machine crankshaft via the rubber belt as normal.
Given that both small gears involved have 17 teeth and both large gears have 40, that’s two successive speed reductions of 40:17 or 2.35:1, so the overall reduction is just over 5:1 or 80%. Readers who did mechanics at school will know that where there’s a speed reduction through a gear train there’s a consequent torque increase, but what that might amount to here in practice is anybody’s guess. Suffice it to say that piercing power is definitely increased by a useful amount.
I think we’ve just about done the speed reducer now except for two last things. One is that the small gear A is plastic, no doubt so that in the event of the drive train getting overloaded, the small gear fails before the motor suffers damage. That had certainly happened with one unit I’ve seen, which had one tooth missing and one a bit dog-eared either side of it, despite which it ran well enough (albeit noisily).
Finally, the speed reducer does need lubrication. I have no idea what the original Elna recommendation was, but given that we have here both steel and plastic parts under variable load in close contact, my own preference is for a synthetic Tri-flow type grease as used by folks into radio-controlled cars and suchlike.
[By way of a footnote, connoisseurs of things mechanical will have noted that with 17 teeth meshing with 40 teeth, the speed reducer was designed on the hunting tooth principle, the aim of which is to distribute wear evenly. And that reminds me - if you're a gear-head, check out the three awesome PDFs by Phil Miller which are linked to at the top of this article ...!]