Sprocket drive unit 250 Nm

Jefa 250 Nm sprocket autopilot drive unit
The Jefa sprocket drive is the most compact sprocket autopilot drive in the world. It's much stronger than a human being (the max. output torque of 250 Nm at 9 RPM is equivalent to 50 Kg on the rim of a 1 meter wheel) and is build for 24 hours per day continuous operation with a total weight of only 7 Kgs. The combination of the specially designed electric motor with the ultra efficient 3 stage planetary gearbox results in an extremely efficient drive unit to keep the battery charging time to the minimum. The drive can be used on boats from 35 to 65 foot (or with a rudder torque up to 400 KgM) equipped with a cable or rod steering system. Please click on the pictures for a larger view.
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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 torque 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 clutch can disengage, making the manual take over much safer. 
The performance

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 manpower. 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 boat 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 long, the pilot will not be able to steer the boat in heavy sees while reaching as it is unable to react quickly 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 smaller 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

1 26 11*1
1¼ 21 11*1
1½ 18 11*1
1¾ 15 11*1
2 13 11*1
2¼ 12 11*1
2½ 11 11*1
2¾ 11 13
3 11 14
3½ 11 16
4 11 18
4½ 11 21
5 11 23
*1: This drive is as standard delivered with a 11 teeth 5/8" sprocket. Please specify when different.
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.
Autopilot junction box 12 Volt version. Max. output (Amp.) Max. rudder torque (KgM)
HO-time no load   12 sec.
HO-time max. load   16 sec.
Simrad AC10 Too small, don't use in combination with this drive 
Simrad AC20 20 171
Simrad AC40 40 395
B&G h1000*² 25 227
B&G h2000 ACP-1 25 227
B&G h2000 ACP-2 40 395
Raymarine S1 Too small, don't use in combination with this drive 
Raymarine S2 Too small, don't use in combination with this drive 
Raymarine S3 30 283
*² Please use part number h1000-CUW specially made for Jefa drives, equipped with dynamic braking. Don't use the standard h1000.
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.
Autopilot junction box 24 Volt version. Max. output (Amp.) Max. rudder torque (KgM)
HO-time no load   12 sec.
HO-time max. load   16 sec.
Simrad AC10 Too small, don't use in combination with this drive 
Simrad AC20 20 390
Simrad AC40 overpowered, do not use
B&G h1000*² 20*¹ 390
B&G h2000 ACP-1 not available in 24 Volts
B&G h2000 ACP-2 20*¹ 390
Raymarine S1 not available in 24 Volts
Raymarine S2 not available in 24 Volts
Raymarine S3 20*¹ 390
*¹ The output of the junction box should be limited to 20 amps. by a fuse in the power input of the junction box (see schematic above, the lines "to battery").
*² Please use part number h1000-CUW specially made for Jefa drives, equipped with dynamic braking. Don't use the standard h1000.
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.
Pricing

Jefa sprocket drive units

Part No.

Description

Price in Euro

DU-WS250-12

Sprocket autopilot drive unit 250 Nm 12 Volts *1

 € 2.263

DU-WS250-24

Sprocket autopilot drive unit 250 Nm 24 Volts *1

 € 2.372

CH5800

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

 € 87

CHLNK

5/8" stainless steel master link

 € 5

DU-WS-BR

Bracket for sprocket drive

€ 116

-

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

 € 15 - € 50

-

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

€ 115 - € 155
*1: This drive is as standard delivered with a 11 teeth 5/8" sprocket. Please specify when different.