Sprocket drive unit

Main dimensions

The Jefa sprocket drive is very compact with a length of 241 mm and a maximum diameter of 178 mm. As space is always a problem on sailing yachts, the compact drive unit can be integrated into the steering system even when a very limited space is available. The drive can operate on any mechanical steering system which is equipped with a rotating shaft. 

The mounting possibilities:

The sprocket drive can be used to drive any system with a rotating shaft. The above example show the autopilot drive driving a second sprocket on the steering shaft.

The construction:

The above assembly drawing shows the cross section of the sprocket drive. The drive can be separated in 3 main parts: The electric motor (1 and 2), the three stage planetary gearbox (3) and the electro-magnetic clutch. The Jefa sprocket drive has multiple advantages over existing sprocket drive units. These advantages will be explained per section of the drive:

Electric Motor: The flat wound electric motor used in the Jefa drive units is not a standard industrial motor, but specially designed and produced for this application. This solution is more expensive but there are multiple advantages over standard industrial motors.

• The collector (1) is extremely big so a large contact surface of the coal brushes is achieved. The result of this is a very high efficiency under heavy load and a very small drop in speed at the maximum torque. (Only 25% loss of speed at full load compared to a stall situation (zero speed) with other brands of autopilot drives).
• The central motor shaft runs in ball bearings in stead of gliding bearings. This results in a higher efficiency, less noise and a much longer life time.
• A large flat wound rotor to achieve a high starting toque and an immediate response to the autopilot speed control signal.
• Specially produced magnets with an extremely long lifetime and field strength. (Normal magnets can loose up to 50% field strength over a couple of years).
• Exchangeable coal brushes to guarantee an infinite lifetime of the drive.
• Aluminium motor housing in stead of sheet steel plate to avoid corrosion.

Planetary gearbox: To achieve a correct rudder travel speed (hard over time) the electric motor has to be reduced in speed with a factor 486:1. Some autopilot drive producers use a worm reduction box, but the efficiency is extremely low as the gears rub each other. Some producers use a combination of a planetary gearbox with a 2 stage spur gear set (one small gear and one big gear), but this creates a much bigger unit with an offset to the centre and much heavier loaded gears. The best way to achieve this high reduction in combination with a high torque is a 3 stage planetary gearbox. It's also the most expensive way as every gear step has 4 gears in stead of 2 gears but the huge advantages overrule the higher price:

• The highest possible efficiency compared to any other gearbox.
• All forces are equally spread over 3 gear teeth in stead of one allowing a much compacter and stronger solution.
• The forces and torques from the motor to the output shaft remain in the centre line of the drive unit, resulting in a higher efficiency and extremely reduces the loads on the housing and other internal parts.

Electro magnetic clutch: On the moment the sprocket steering system on the yacht is manually operated, the autopilot drive has to be disconnected from the steering system. This is achieved with the electro-magnetic clutch (4) and controlled automatically by the autopilot junction box. Jefa has developed a unique and patented engagement clutch. The solution is based on two electrically operated spring loaded clutch pins that engage and disengage the outer gear ring of last planetary gear step. This solution has multiple advantages over the existing friction plate clutches:

• Less friction to back drive the unit.
• Lower power consumption (1.3 Amp. at 12 Volt, 0.7 Amp at 24 Volt). When the clutch isn't powered, it's disengaged and engaged when powered.
• The clutch doesn't wear in time.
• More compact than any friction clutch.
• When the autopilot is switched off, the helmsman isn't suddenly confronted with the full rudder torque, but has to put loading on the wheel to equalise the forces so the the clutch can disengage, making the manual take over much safer. 

This performance table shows the relation between the consumed power and the output power. The red line shows the output torque against the needed amperage for the 12 volt unit and the green line for the 24 volt unit. The blue line shows the output speed of the drive relative to the output torque. Also visible is the strength of the drive unit related to man power. The unit is much stronger than a human being and can last much longer but one should note that when the unit is operated in the red zone, something is wrong with the trim of the boats and the sails should be adjusted to achieve lower rudder torques. The above table shows that the Jefa sprocket drive will steer the yacht even in the worst possible conditions. As the drive will mostly operate in the left green zone and will not continuously rotate, the average power consumption on 12 volts is 4 amps and on 24 volts 2 amps.

Connections:

This illustration shows the minimal components for a working autopilot configuration. Jefa autopilot drives work together with all mayor autopilot electronics. The connection of the Jefa autopilot drive to the autopilot junction box is quite simple. The two 1.5 mm² red and black wires have to be connected to the plus and minus of the autopilot clutch line. This will make sure that when the autopilot user engages the autopilot on the control screen, the clutch will engage and allow the autopilot motor to drive the steering system. The two 4 mm² red and black wires have to be connected to the autopilot drive output connection. Now the Jefa autopilot drive is integrated into the system.

How to achieve the correct hard over time (autopilot speed) by varying the sprockets.

Following table shows the correct sprocket ratios to achieve the correct hard over time (the time needed for the autopilot drive to run through 72° of rudder travel). The ideal hard over time is between 12 and 15 seconds. As most modern autopilots have a feature called "speed control" (the speed of the autopilot can be step less controlled by pulse width modulation) it's better to aim at the 12 seconds as the autopilot can always slow down the drive when the response is too quick. It's very important to get the sprocket ratio right as a too quick autopilot drive (less than 10 seconds HO time) will be too nervous and will cause the autopilot to start swinging between port and starboard (officially named "hunting"). When the hard over time is too low, the pilot will not be able to steer the boat in heavy sees while reaching as it is unable to react quick enough to the changing courses. The working order is as follows: First count the amount of turns lock-lock of the driving shaft (the shaft the sprocket drive will drive) assuming the rudder travel is around 72°. (If the rudder travel is more than 72°, multiply the counted turns with 72 and divide it through the measured angle). If the counted turns are more than 2.5, the drive has to be speeded up, so the drive shaft sprocket has to be bigger than the drive unit sprocket. When the counted turns are below 2.5, the drive has to be slowed down by using a bigger sprocket on the drive shaft. Jefa can supply 5/8" pitch stainless steel sprockets bored and keyed to suit every drive shaft and matching 5/8" stainless steel chain.

Jefa sprocket drive calculation

Counted turns of drive shaft

Drive shaft sprocket

Autopilot drive sprocket

Compatibility in 12 Volts.

Following table shows the maximum rudder torque that can be generated by the Jefa 250 Nm sprocket drive in combination with various autopilot junction boxes. As the sprocket drive can generate much more torque than a human being, one should test the system after installation by blocking the rudder movement. This way one can determine if every part of the steering system can withstand the loading.

Compatibility in 24 Volts.

Following table shows the maximum rudder torque that can be generated by the Jefa 250 Nm sprocket drive in combination with various autopilot junction boxes. As the sprocket drive can generate much more torque than a human being, one should test the system after installation by blocking the rudder movement. This way one can determine if every part of the steering system can withstand the loading.

Mounting bracket for flat mounting:

When mounting on a surface parallel to the chain is not possible, an additional mounting bracket can be fitted to allow mounting on a flat surface. The slotted holes in the drive flange can be used for ease of tensioning of the chain in horizontal or vertical direction.

Jefa sprocket drive units

Part No.

Description

DU-WS250-12

Sprocket autopilot drive unit 250 Nm 12 Volts

DU-WS250-24

Sprocket autopilot drive unit 250 Nm 24 Volts

CH5800

5/8" non-magnetic stainless steel chain per meter

CHLNK

5/8" stainless steel master link

DU-WS-BR

Bracket for sprocket drive

-

Various stainless "plate" sprockets to be welded on shaft 11T - 26T

-

Various stainless "hub" sprockets bored and keyed to suit shaft 11T - 26T