Data Sheet

RB-Sct-828
ServoCity 7:1 Top-mount Gearbox for HS-785HB Servo
The SPG785A Servo Power Gearbox utilizes the Hitec HS-785HB sail winch servo (included) to create
a high-torque Servo Power Gearbox. The stock Hitec servo can rotate several turns in factory form
(approximately 4 turns on most Tx/Rx systems; up to 8 turns with a 650-2350usec signal). When the
servo is installed into the gearbox, the total amount of rotation is decreased by the ratio selected but the
torque and precision is increased by the ratio. The lower the gear ratio, the more precision and torque it
will provide but the speed and amount of rotation will be decreased. Five different gear ratios are
available in order to find the right balance between torque and rotation. Positioning feedback remains
since the servo is completely stock and unmodified in any way. The SPG785A can handle tremendous
side loads with the dual ABEC-5 precision ball bearings which support the hardened 3/8” stainless steel
shaft. The shaft is hollow with a .290” inner diameter to allow wires from cameras, sensors or other
devices to not tangle during multi-rotation movements. The brass pinion gear meshes to a high-grade
7075 aluminum 32 pitch hub gear which is fastened to the output shaft of the gearbox with a 3/8”
clamping hub. The .770” clamping hub has 6-32 tapped holes which allow easy attachment of various
components. The frame of the SPG785A is machined from 6061-T6 aluminum (anodized) and can be
mounted using the top-mount tabs which are ¼” thick and have .140” thru-holes (allows a #6 screw to
be used).
Specifications
• SPG785A Servo: HS-785HB (7:1 ratio)
• Rotation: 180°
• Weight (With Servo): 9.4 oz.
• 4.8V Power (ounce-inch): 1,071
• 6.0V Power (ounce-inch): 1,281
• 4.8V Speed (sec/60°): 1.96 sec.
• 6.0V Speed (sec/60°): 1.61 sec.