Chapter 1. 

Basic Differences

Overview

The SG (Simple Governor) has been an important product line of Woodward’s extensive mechanical  hydraulic governor product offerings over the years. The SG governor has served the market for diesel  engines, gasoline engines, steam turbines, or gas turbines since the early 1940’s. However, after more  than 80 years of availability, the market demand for new SG governor applications has diminished to the  point that Woodward can no longer manufacture this product line with cost effectiveness. With options for mechanical upgrades to the PSG (see Application Note 51650), as well as electronic governing systems with EG-3P, Woodward has decided to inactivate and stop all production of SG governor moving forward. EG-3P actuators are electronically controlled actuators which utilize oil pressure (either generated internally or supplied externally) to output torque across their 42° range. With many models available that match SG base and SG driveshaft options, the EG-3P is a popular option for customers who wish to upgrade to an electronic control system. 

The basic governor function of the SG governor is based on a fixed droop setting, resulting in engine  speed reduction (drooping) when load on the engine is increased. The EG actuator series provides a rotary output proportional to the electrical signal that it receives from a controller. The controller can operate in isochronous mode or fixed droop mode depending on how the controller is set up. Switching your engine from an SG governor to a EG actuator requires a controls system upgrade and consulting applications engineer to check the SG governor configuration and possible engine specific settings. Based on these parameters, a specific controller can be suggested along with the necessary speed sensors, typically using a couple MPUs (Magnetic Pickup). 

Chapter 2.
SG Governor Overview

The SG governor is a hydraulic speed droop governor that is used on diesel engines, gas engines, steam turbines, or gas turbines where isochronous (constant speed) control is not required. The design of a droop governor is such that as load is increased, the desired speed of the governor is decreased, achieving stability in the governed system.  

SG governors are supplied with a useful net torque output of 11.3 and 28.8 in-lbs. over a terminal shaft travel of 36°. A return spring (internal or external and vertical or horizontal) is used to move the power piston toward minimum fuel. Therefore, the actual torque developed during a speed decrease is 11.3 in lbs. or 28.8 in-lbs. plus the return spring force. During an on-speed condition, the force developed by the power piston is equal to the return spring force. The relief valve spring used determines whether the unit has a net torque of 11.3 in-lbs. or 28.8 in-lbs.

Normally, the SG governor operates at speeds between 2400 and 3600 rpm but special configurations are available to meet other speed requirements.

The unit will operate vertically or horizontally. In the horizontal position, the mount must be such that the terminal shaft is also horizontal and requires a drain on the lower side of the cover.

Speed Adjustment

Several options are available to adjust the desired speed of the governor.

Speed Setting Shaft A speed adjusting shaft may be used to change the force on the speeder spring (desired speed). Stop screws can be used for limiting the speed range of the variable speed governor.

Speed Adjusting Motor Motors may be fitted on the governor cover to enable an operator to adjust the speed of the governor from a remote location. Several types of motors are available: Groschopp, Pittman, and Sawamura types. 

 • Groschopp motor is a split field, series wound, reversible motor. Motors are available in the following voltages: 115V AC/DC, 125V DC, 64V DC, 48V DC, 32V DC, 24V DC, and 12V DC. 

 • Pittman motor is a permanent magnet motor and only available for supply voltages of 12V DC and 24V DC.

Pneumatic Speed Adjustment The pneumatic speed setting assembly is used to change the speed setting of the governor from a remote location and can be direct acting or reverse acting. The pneumatic speed setting mechanism replaces the screws normally used to set high and low speeds. The most common range is 3-15psi, but 6-30psi, 9-15psi, 3-20psi, and 5-60psi ranges are available.

The pneumatic speed setting assembly consists of a diaphragm, housing, oil reservoir, return spring, and a pushrod that extends to the floating lever in the governor. Speed will change proportionally to the change in supply air pressure. As an example, assume that the operator wants to increase the speed of the engine. The desired speed is increased by the operator increasing the air pressure to the reservoir— oil is forced through the orifice to the top of the diaphragm. As the diaphragm and piston move downward, the upward force of the spring is increased as the spring is compressed, re-establishing the force balance. The downward movement of the rod extending to the floating lever increases the speeder spring force (desired speed) and thus increases speed.

Pump and Relief Valve

Supply oil (usually engine oil pressure) is supplied to the oil pump. Oil is carried in the gears around the outside of the gear pocket to the pressure side. As pressure increases, the relief valve plunger moves against the relief valve spring until the plunger exposes the hole allowing pressurized oil to return to sump.

Ballhead, Pilot Valve, Bushing, Speeder Spring Assembly

This assembly allows the governor to compare desired speed to actual speed and make a decision whether to add, decrease, or make no change to the terminal shaft position, which controls fuel to the engine. A force balance exists between the ballhead and the speeder spring. The speeder spring force is the desired speed force and the centrifugal force of the ballhead (actual speed) acting opposite to the downward force of the speeder spring. During an on-speed condition, the forces balance and no oil is ported to or from the power piston. If the forces are not equal, pressurized oil will be ported to or from the power piston, resulting in a change in terminal shaft position. The bushing rotates, driving the ballhead at a speed proportional to the engine speed. Its rotation reduces the frictional forces between the pilot valve and bushing. Different speeder springs, bushings, and ballheads are available for variations of speed and engine response.

Power Piston

The power piston provides the force to move the terminal shaft to increase or decrease fuel to the engine. The piston is single acting, only exerting hydraulic force in the increase direction. When the pilot valve is lowered in the increase fuel direction, governor pressurized oil is ported through the rotating bushing to the power piston, which moves upward. In a decrease fuel direction, the pilot valve raises allowing trapped oil to move from the power cylinder through the bushing to drain. The downward force that causes this movement comes from the return spring. The force applied to the lower side of the power piston during steady state operation is equal to the force applied by the return spring. In an increase transient or at blocked servo at max fuel the pressure will be equal to that generated by the pump plus the supply pressure. At minimum fuel and blocked servo, the servo pressure will be zero with only the return spring force urging the power piston to minimum.

Speed Droop Adjustment

Droop is defined by an increase in load and a resulting decrease in speed setting. This is quantified as the change in rpm between maximum fuel and minimum fuel. Droop accomplishes three things:   

1. Stability 2. Load limiting 3. Ability for multiple engines driving a common load to share load

The SG governor accomplishes this with a speed droop bracket and lever. As load increases and the power piston moves upward, the speed droop bracket and lever raise, lifting the floating lever which decreases the force on the speeder spring, thus lowering the desired speed of the engine. As the droop slider is moved away from the terminal shaft, the amount of droop is increased.  

A minimum amount of droop (0.5% to 7%) is necessary to stabilize an engine operated with a governor without a compensation system. If negative droop is present, the governor will be unstable.

Example of Operation

Assume that the operator wants to increase the speed of the engine, or an additional load is added to the system. The governor responds by:  

1. The ballhead moves inward allowing the pilot valve to move downward below its “null” position. 2. Pressurized oil is ported through the bushing to the power piston moving it upward. 3. The terminal lever rotates clockwise increasing fuel to the engine. 4. As the engine accelerates from the added fuel, the flyweights move outward, lifting the pilot valve to its “null” position and stopping the increase of fuel.  

   The high-speed stop screw limits the top end speed of the engine and the low-speed stop screw controls the low speed. Setting these two controls the operating range of the engine.

   
   

   
   

   
   

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