Why do mowers and skid loaders use a hydrostatic pump
 

As the title suggest, why is it that skid loaders need to use hydrostatic pumps to run the drive motors. Why cant you just use a standard gear pump? From my understanding the hydrostatic pumps have a variable displacement, i assume to control speed at a certain engine RPM. A gear pump flows x amount at 1000 rpm and raises as engine speed increases. Now couldnt you use one of these to power a drive motor and use a spool vlave to control the speed? Im in the process of planning a very small loader for a small shop.

So why do i need a hydrostatic pump?

I have a Ramrod 900 skid steer that you ride on the back of, it has two hydraulic pumps both of which are gear type. One pump is high flow while the other low flow and you can select between either. The low flow is good for training or maneuvering in tight spaces but once you get the feel of the machine hi flow can be used for most things. Max pressure is 3000psi with a pressure relief at 3250psi. The one draw back with the machine is it mostly comes with a 20HP Briggs Vee twin but I recently changed it out for a Honda 20HP vee twin and what a difference it made. These machines can be had quite cheaply in the US and I would suggest changing out the engine. That Honda engine was half the price in the US but no one would ship to Canada. The engine was a direct bolt in and all the controls are mounted on the engine side panel along with an hour meter, the fuel also hooked straight in. As I run a larger battery I had to move the battery box forward a little along with the original muffler. The Honda engine is the GX630QXF and is around $1100 in the US.

so your saying your Mini skid does infact just use a gear pump. Interesting. Im not a hydraulic major but in my mind it seems to work. Using valving to limit flows ect, Im aware this isnt as efficient but should suffice for smaller usage as i wont use it daily

So can someone tell me why you would WANT to use a hydrostatic pump?

My hydraulic knowledge comes from aircraft so somewhat different so I looked this up.

The simplest form of hydrostatic transmission uses a fixed-displacement pump driving a fixed-displacement motor, Figure 3A. Although this transmission is inexpensive, its applications are limited, primarily because alternative forms of power transmission are much more energy efficient. Because pump displacement is fixed, the pump must be sized to drive the motor at a fixed speed under full load. When full speed is not required, fluid from the pump outlet passes over the relief valve. This wastes energy in the form of heat.

Using a variable-displacement pump instead of one with a fixed displacement creates a constant torque transmission, Figure 3B. Torque output is constant at any speed because torque depends only on fluid pressure and motor displacement. Increasing or decreasing pump displacement increases or decreases motor speed, respectively, while torque remains fairly constant. Power, therefore, increases with pump displacement.

Using a variable-displacement motor with a fixed-displacement pump produces a transmission that delivers constant power, Figure 3C. If flow to the motor is constant, and motor displacement is varied to maintain the product of speed and torque constant, then power delivered is constant. Decreasing motor displacement increases motor speed but decreases torque, a combination that maintains constant power.

I use my machine 90% of the time at just off idle.

Probably a lot easier on shafts and joints, too, and less areas to get stuff plugged up in.

The main reason for that type of system is that modern equipment use an open center hydraulic system. That type of system allows you to control the system based on the resistance in the motors and the cylinders. The more resistance, the more pressure you apply to achieve the desired action. I'm going to include a link to wikipedia but I encourage you to Google open center hydraulics. There are several good articles on the net that will explain it in as much detail as you want.

http://en.wikipedia.org/wiki/Hydraulic_machinery

Besides using open center pumps many of the newer loaders, excavators, etc store the hydraulic pressure in an accumulator when the work load is light, like with a full bucket going down.
They also throttle down to idle when the stored pressure is being used.
Up to a 25% reduction in fuel usage.

With simple gear pumps and spool valves it's pretty much 1 thing at a time,
the first valve open hogs most of the power.