HYDROSTATIC TRANSMISSIONS

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

HYDROSTATIC TRANSMISSIONS
SERVICING HYDROSTATIC TRANSMISSIONS
TROUBLESHOOTING A HYDROSTATIC TRANSMISSION
HYDROSTATIC TRANSMISSION TROUBLESHOOTING CHART

HYDROSTATIC TRANSMISSIONS

Hydrostatic transmissions often used on garden tractors. Instead using gears and shafts to transmit power, hydrostatic uses motor to move pressurized fluid through system of lines. Fluid pressure transmits power from engine to drive wheels of machine. Power produced by pressurized fluid called hydraulic power.

In typical hydrostatic drive, rotary power from engine turned into hydraulic power by pump connected directly to engine crankshaft. Motor then converts the hydraulic power back to mechanical rotary power that can be transferred to drive wheels.

Because flow hydraulic power easily started, stopped, and controlled, hydrostatic drive can replace clutch and transmission in power train. Can produce infinite speed and torque changes from full forward to full reverse. This permits operator to control drive speed of machine without affecting speed of auxiliary attachments like mowers, sprayers, etc. Components hydrostatic transmission are hydrostatic pump, control mechanism, and hydraulic motor all combined in single housing. Most cases, hydraulic motor receives power from engine through either drive shaft or belt drive. Solid pipes or flexible hoses carry high pressure fluid from pump to hydraulic valves, then to hydraulic cylinders that push or pull linkages. Pushing or pulling force used to drive wheels. Power from hydrostatic transmission may also run hydraulic lift system or power steering.

Hydrostatic pump positioned at in put end of transmission. Pump is actually small cylinder block that contains small pistons. These pistons drive flow fluid to output motor. Output shaft splined into output motor’s cylinder assembly, and changes its direction of rotation with output motor. The shaft then drives the differential.

Hydraulic motor has fixed displacement, a set volume fluid required to turn motor’s output shaft. The larger the volume of oil pumped to motor, the higher the pressure that pushes on motor cylinders. The more pressure that pushes on hydraulic motor’s cylinders, the quicker the motor’s output shaft turns.

Amount fluid allowed to flow through system controlled by a swash plate. Swash plate can be angled back and forth to control flow fluid through system. Angle of plate controls amount and direction of fluid flow through system, allowing transmission to have different speeds (plus forward and reverse positions). Reverse on hydrostatic transmission achieved by reversing flow fluid through system.

When swash plate in vertical position, no fluid flow and transmission in neutral. When swash plate angled 15 degrees forward, transmission moves into forward. When plate angled to 7 degrees backwards, transmission moves into reverse.

Different speeds obtained by increasing swash plate angle. As forward angle plate increases, more fluid forced toward output motor. Because output motor displacement fixed, must rotate faster so pistons can accept increased flow. Thus forward speed tractor increases. Operator controls direction swash plate rotation through mechanical linkage.

In hydrostatic system, charge pump directs fluid flow to make up for internal leaks in system and to circulate fluid for cooling purposes. Charge pump usually belt driven by engine. Charge pressure regulated by charge pump relief valve. Two check valves regulate distribution of replacement fluid to parts of system that need it most. Surplus fluid overflows these valves and is stored in reservoir. Some models also have acceleration valves to control response rate transmission. All these valve positions factory set.

Are some variations to type hydrostatic transmission described. Some models have gear type motor instead of piston, and some are equipped with high pressure relief valves. Units that don’t have high pressure relief valves rely on wheel slippage to occur as pressures get too high. Output shaft in this type hydrostatic unit connected to a differential that drives wheels.

Speed of hydrostatic transmission infinitely variable, enabling operator to choose output rpm from zero to high speed. Hydrostatic transmissions usually cost more than mechanical transmissions, but offer several advantages over gear transmissions. Main advantage is that movement one lever can change speed and direction of machine. This eliminates need for clutch and gear shift. Transmission speed can be changed at any time without stopping. Shifting is smooth, without steps, and high amount torque available at startup. Hydrostatic transmissions compact and require less maintenance and service than other types transmissions.

Hydrostatic transmissions sometimes used on riding mowers, but seen more on garden tractors. Hydrostatic transmission is motor that transfers power by moving pressurized fluid within closed loop. Hydrostatic transmission contains pump, a control mechanism, a motor combined in single housing. Input end transmission receives power from engine through a drive shaft or from a belt drive. Solid pipes or flexible hoses carry high pressure fluid from pump to hydraulic valves, then to hydraulic cylinders that push or pull linkages. Pushing or pulling force used to drive wheels. Power from hydrostatic transmission may also be used to run hydraulic lift system or power steering.

Hydrostatic pump positioned at input end transmission. Pump is small cylinder block assembly that contains small pistons. Pistons drive flow fluid to output motor. Output shaft splined into output motor’s cylinder assembly, and changes its direction rotation with output motor. Shaft then drives differential.

Amount fluid allowed to flow through system controlled by a swash plate. Swash plate can be angled back and forth to control flow fluid through system. Angle of plate controls amount, direction fluid, thus allowing transmission to have different speeds, plus forward and reverse positions. Reverse on hydrostatic transmission achieved by reversing flow fluid through system.

When swash plate in vertical position, no fluid flow. This neutral position. When swash plate angled to 15 degree forward, transmission moves into forward. When plate angled to 7 degree backward, transmission moves into reverse.

Different speeds obtained by increasing swash plate angle. As forward angle plate increases, more fluid forced toward output motor. Because output motor displacement fixed, must rotate faster so pistons can accept increased flow. Thus, forward speed tractor increases. Operator controls direction swash plate rotation through mechanical linkage.

The speed control this type transmission infinitely variable, operator can choose any output rpm from zero to full speed depending on conditions. Engine rpm may be adjusted to suit most efficient operating range, or to appropriate speed for operating attachments.

Main advantage hydrostatic transmission over gear transmission is movement one lever can change speed and direction vehicle. This eliminates need for clutch and gear shift. Transmission speed can be changed without stopping or declutching.

In hydrostatic system, charge pump directs fluid flow to make up for internal leaks in system and to circulate fluid for cooling. Charge pump usually belt driven by engine. Charge pressure regulated by charge pump relief valve. Two check valves regulate distribution replacement fluid to parts system that need it most. Surplus fluid overflows these valves, is stored in reservoir. Some models have acceleration valves to control response rate transmission. All valve positions factory set.

Some variations to the type hydrostatic transmission here described. Some models have gear type motor instead of piston, some units equipped with high pressure relief valves. Units that don’t have high pressure relief valves rely on wheel slippage to occur as pressures get too high. Output shaft in this type hydrostatic unit connected to a differential that drives the wheels.

SERVICING HYDROSTATIC TRANSMISSIONS

Most hydrostatic transmissions don’t require lot maintenance. Oil and filters must be changed at regular intervals recommended by manufacturer. Important to keep hydrostatic transmission system clean to protect against contamination dirt or water. Always use proper lubricant and filters as recommended by manufacturer.

To replace oil and filter in hydrostatic system, clean exterior machine and outside transmission components. Then lift machine as needed to gain access to transmission. Remove oil drain plug and drain old oil into container. After oil drained, replace plug. Remove filter and clean seating area thoroughly. Check old filter for presence tiny metal particles. These particles indicate internal transmission components wearing. Install new filter according to manufacturer’s specifications. Remove oil fill plug, and add proper oil to level specified in service manual.

Like gear type transmissions, some hydrostatic can be disassembled and rebuilt or repaired. Decision whether to rebuild will depend on machine model, cost, time involved, and availability parts. Even when hydrostatic transmission disassembled and rebuilt, hydrostatic pump shouldn’t be disassembled unless service manual specifies. Hydrostatic pumps contain myriad small parts like ball pistons and springs that are carefully assembled in factory to precise specifications. Difficult to take pump apart and reassemble to exact same specifications. If hydrostatic pump reassembled incorrectly, transmission won’t work. If pump has problem, will usually have to be replaced as unit. Never attempt to disassemble hydrostatic pump, its motor, or control system unless have adequate training, proper tools, and highly clean work area. Always use clean workbench or table when disassembling pump or motor for service. Use special fixture to hold hydrostatic transmission during disassembly.

Before begin disassembly, thoroughly clean outside transmission housing. Can use steam cleaning machine, pressure washer, or solvent and clean rags to clean. If use steam cleaner or pressure washer, careful to prevent any water getting into system. Make sure all hose connections tight before washing outside transmission.

During disassembly, keep parts transmission very clean. Never perform internal service work on shop floor or the ground, where danger dust or dirt being blown into parts. Before handling internal transmission components, wash hands and cover them with same hydraulic oil used in system. Be sure all tools clean, free of grease and dirt.

As disconnect each hydraulic line or hose from transmission, use clean plastic bags to cover and seal openings. Plastic bags will prevent dirt getting into system. Fasten bags securely with rubber bands or tape. Have container clean solvent ready to wash internal components transmission. Use only clean compressed air or lint free cloths to dry parts.

After parts thoroughly cleaned, inspect for wear or damage. Check all manufacturer’s specifications listed in service manual. Replace any parts that worn or damaged, then reassemble transmission. Coat new parts with clean, new hydrostatic fluid before you install. Replace all seals and gaskets with new. Make sure all shafts and other parts move freely in the pump and motor before you reinstall transmission in machine. When reinstall transmission, tighten housing bolts or screws to manufacturer’s specified torque.

After transmission reinstall, fill with clean new hydrostatic fluid per manufacturer’s recommendations. Always install new filter when repairs made. Make sure oil has moved completely through system before engage hydrostatic transmission. Following are some precautions should follow when repairing hydrostatic transmissions:

–Never work on hydrostatic transmission with engine running unless specifically instructed to do so in manufacturer’s service manual.
–Never operate hydrostatic transmission without oil, even for short time. Always check oil level before service or troubleshoot system.

Following is basic troubleshooting chart for hydrostatic transmissions. Most the procedures described can be performed without removing transmission from machine. If transmission does need to be removed for repairs, consult service manual for machine to determine removal procedure. Procedure will vary widely from one model to another. Service manual will also provide detailed instructions for testing components hydrostatic system:

TROUBLESHOOTING A HYDROSTATIC TRANSMISSION

Like gear type transmissions, some hydrostatic can be disassembled and rebuilt or repaired. These usually found in larger, more expensive tractors. The decision about rebuilding or replacing depends on tractor model, cost, time involved, and availability parts. Even when hydrostatic transmission disassembled and rebuilt, hydrostatic pump shouldn’t be disassembled. Hydrostatic pumps contain many small parts like ball pistons, springs, etc. that carefully assembled in factory to precise specifications. Is very difficult to take one apart and reassemble to same specifications. If hydrostatic pump reassembled incorrectly, transmission system won’t work. If determine hydrostatic pump has problem, should be replaced.

Below is troubleshooting chart for hydrostatic transmissions. Most procedures referenced can be performed without removing transmission from tractor. If transmission does need to be removed, need to consult service manual to determine removal procedure. Procedure will vary widely from model to model. Service manual will also provide instructions for testing components hydrostatic system:

HYDROSTATIC TRANSMISSION TROUBLESHOOTING CHART

A)If The Machine Loses Power Or Won’t Move In Either Direction
1)The oil may be low
2)The wrong kind of oil may have been used
3)The system may have an oil leak
4)There may be a restriction in the oil line
5)The filter may be plugged
6)The drive may be disconnected
7)The control linkage may not be functioning
8)The pump and motor may be damaged or worn out
9)The charge pressure may be too low
10)The check valves may be defective
11)The charge pump relief valve may be malfunctioning
12)The charge pump may be defective
13)The pump drive shaft may be binding
14)The system may have air in it
15)The pressure control valve may be stuck open
16)The check valves may be faulty
B)If The Machine Jerks Or Will Only Move In One Direction
1)The fluid level may be low
2)The filter may be plugged
3)The throttle linkage may be damaged
4)The directional check valve may be leaking
5)The high pressure relief valve may be faulty
6)The one dampening piston may be leaking
7)The check valves may be defective
C)If The Machine Doesn’t Stay In Neutral
1)The linkage needs to be adjusted
2)The displacement control valve may need to be adjusted
3)The pump servo valves may need to be adjusted
D)If The Transmission Runs Hot
1)The fluid may be low
2)The wrong type of fluid may have been used
3)The filter or the cooling fins may be plugged
4)The fan may be damaged
5)The system may be overloaded
6)The check valves may not be working properly
7)The charge pressure may be incorrect
8)The pump and motor may need to be repaired or replaced
9)The charge pump relief valve may need to be replaced
10)The engine fan belt may be slipping or broken
11)The high pressure relief valve may be stuck open
E)If The System Is Noisy
1)There may be air in the system
2)There may be a lack of oil
3)The wrong oil may have been used
4)The pump or motor rotating group may be damaged
5)The pump or motor may be worn
6)The lines or hoses may be damaged
F)If The Transmission Leaks Oil
1)The case may be loose or cracked
2)The shaft seals may be leaking
3)The hose connections may be loose, leaking, or damaged
4)The lines or seals may be leaking
5)The O rings or seals may be damaged
G)If Acceleration And Deceleration Are Sluggish
1)The pump or motor may be worn or damaged
2)There may be air in the system
3)The wrong fluid may have been used
4)The oil supply may be low
H)If The System Speeds Up During Operation
1)The control linkage may be faulty
I)If The Transmission Is Hard To Shift Or Will Not Shift
1)The speed control lever may not be in neutral
2)The gearshift linkage may be out of adjustment

Here are some general guidelines to follow when disassembling and servicing hydrostatic transmissions. Consult service manual for garden tractor to obtain specific instructions:

1)Use special fixture to hold hydrostatic transmission during disassembly. Typical holder for hydrostatic transmission will be rectangular block with dimensions 10 ½ inches length x 3 ½ to 4 inches width x minimum 1 ½ inches height. Will contain two holes. First hole has diameter 2 ½ inches, will be centered across width block, and center of hole 2 ½ inches from end of block. Second hole 1 ½ inches in diameter, will be centered across width block, and center of hole 4 inches from center larger hole.
2)Before begin disassembly, thoroughly clean outside transmission housing. During disassembly, keep parts transmission very clean.
3)Before handling internal transmission components, was hands and cover them with same hydraulic oil used in system.
4)Check filters for metal particles. Particles in filter indicate transmission components wearing.
5)Check manufacturer’s specifications found in service manual before reassembly.
6)Check parts for wear, and replace as necessary.
7)Before reassembly, cover all parts with correct hydraulic oil.
8)Make sure all seals and gaskets replaced with new.
9)Tighten housing bolts or screws to manufacturer’s specified torque.
10)Prime unit with oil before reinstalling in tractor.