{"id":82770,"date":"2026-10-26T12:48:19","date_gmt":"2026-10-26T12:48:19","guid":{"rendered":"https:\/\/hydnet.se\/?p=82770"},"modified":"2026-10-26T12:48:19","modified_gmt":"2026-10-26T12:48:19","slug":"hydrostatic-drivelines-for-off-road-machines-how-to-size-pump-motor-and-gearing","status":"publish","type":"post","link":"https:\/\/hydnet.se\/en\/hydrostatic-drivelines-for-off-road-machines-how-to-size-pump-motor-and-gearing\/","title":{"rendered":"Hydrostatic drivelines for off-road machines \u2013 how to size pump, motor and gearing"},"content":{"rendered":"
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Hydrostatic drive is found in almost every mobile off-road machine: forwarders and harvesters, tool carriers, compact loaders, rollers, mining and tunnelling vehicles. The prime mover is usually a diesel engine, but it can just as well be an electric motor. The same principle is also used in industrial and marine applications, for example in winches and other deck machinery, where an electric motor is often the natural choice. The principle is simple, but sizing is what makes a driveline good or bad. Pump, motor, gear ratios and prime mover have to match. Otherwise the machine ends up with too little tractive force, too low a top speed, an engine that stalls on the hill or oil that boils after an hour of driving.<\/p>\n

This article shows how we usually lay out and calculate a classic hydrostatic-mechanical driveline for an eight-wheeled off-road machine. All figures in the worked example are typical values<\/strong> chosen to show the method. Your machine will have its own numbers. The worked example is based on a diesel engine. With an electric motor the method is the same, only the power and torque curve looks different. At the end you will find a checklist of the information we need to prepare a first proposal for you.<\/p>\n

How the driveline is built<\/h2>\n
\"Schematic
Figure 1. Hydrostatic-mechanical driveline: one pump, one motor and mechanical power transmission to all eight wheels. Red is the high-pressure side and blue the low-pressure side of the closed circuit. The sides swap when the direction of travel is reversed.<\/figcaption><\/figure>\n

The driveline consists of a hydrostatic part and a mechanical part:<\/p>\n

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  1. Hydrostatic part: pump and motor.<\/strong> The diesel or electric motor drives a variable axial piston pump<\/a>, the hydrostatic pump, in a closed circuit. The pump supplies oil to a single variable hydraulic motor<\/a>. This gives stepless speed control in both directions and full tractive force from standstill. If needed, a pump gear can be fitted between the prime mover and the pump, so that the pump speed can be chosen freely relative to the speed of the diesel or electric motor.<\/li>\n
  2. Mechanical part: transfer gearbox.<\/strong> The hydraulic motor is flanged to a transfer gearbox, often with two mechanical ranges (high\/low). From there, propeller shafts run to the front and rear axles.<\/li>\n
  3. Differentials.<\/strong> Power enters the axles through a crown wheel and pinion, normally with a mechanical differential lock.<\/li>\n
  4. Bogie boxes.<\/strong> The bogie boxes sit at the outer ends of the axles. Inside, a gear train distributes the drive from the axle to both wheels on the bogie. With two bogie boxes at the front and two at the rear, all eight wheels are driven mechanically and in sync.<\/li>\n<\/ol>\n

    The total ratio from hydraulic motor to wheel is the product of the three stages: itot<\/sub> = if<\/sub> \u00b7 id<\/sub> \u00b7 ib<\/sub><\/strong> (transfer box \u00b7 differential \u00b7 bogie).<\/p>\n

    Why one central motor and mechanical distribution?<\/h2>\n