The Hydrostatic Transmission: A Gearbox Made of Oil

Sheet 1 of US 924,787 shows the whole machine in section: two cylindrical casings bolted either side of a mid-plate 13, a cylinder barrel 29 turning against each face of it, and pistons 31 driven by rods 34 from a socket ring 23 on a tilting box 20. The specification's key points: the mid-plate's two ports are extended into tapering V-shaped channels "for the purpose of securing a more gradual opening and closing" as each cylinder port passes them — the same trick used in every axial-piston machine since to stop a pressure shock at each piston. Turning handwheel 51 turns a scroll disk that tilts the box; "when the race 22 is perpendicular to the shaft, the pistons 31 will not move at all in the cylinders 30, and the barrel 29 will rotate idly. The greater the inclination of the race 22, the longer will be the stroke of the pistons 31, and the greater therefore the speed of the driven member." Oil drilled through the pistons and rods lubricates both ball ends.

Screw a plywood disc 120 mm across to the output shaft of a geared DC motor, and drill a row of holes along one radius at 0, 10, 20, 30 and 40 mm from the centre. A pin in one of the holes drives a connecting rod to the plunger of a 20 mL syringe: that is the pump, and the hole you choose is its swash angle. Join its tip with a short, stiff tube to a second 20 mL syringe mounted along a steel rule: that is the motor, and its plunger's travel is the output. Fill both with water and bleed out all the air. Run the motor at a steady speed and measure the output stroke with the pin in each hole. It rises in proportion to the pin radius, and with the pin at the centre the input turns and the output stands still. Change the output syringe for a smaller one: its stroke grows by the ratio of the areas — the motor's displacement sets the ratio too.

**Reynold Janney**, of New York, assignor to the **Waterbury Tool Company** of Waterbury, Connecticut, filed **US 924,787** on 9 July 1906; granted 15 June 1909. With **Harvey D. Williams** he developed the Williams–Janney variable speed gear, among the first hydraulic machines to run on oil rather than water. The US Navy used Waterbury gears to train and elevate gun turrets, and a Williams–Janney gear built in England is in the Science Museum Group collection. Hydrostatic drives now move combine harvesters, skid-steer loaders, lawn tractors and winches. **Honest limits.** The notebook's efficiencies are typical figures, not Janney's: real losses depend on speed, pressure and oil viscosity. A closed loop heats its own oil and needs a charge pump and cooling, which the syringe model cannot show.