Sprocket drive unit 200 Nm 12 volts

Jefa 200 Nm sprocket autopilot drive unit
The Jefa sprocket drive is one of the most compact sprocket autopilot drives in the world. It's as strong as a human being (the max. output torque of 200 Nm is equivalent to 40 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 30 to 50 foot (or with a rudder torque up to 165 KgM) equipped with a cable or rod steering 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), the three stage planetary gearbox (2) and the electro-magnetic clutch (3). The Jefa sprocket drive has multiple advantages over existing integrated drive units. These advantages will be explained per section of the drive:

Electric Motor: The flat wound electric motor (pancake motor) used in the Jefa drive units is carefully selected for this application. Pancake motors have multiple advantages over normal electric DC motors:

  • A large flat wound rotor to achieve a high starting torque and an immediate response to the autopilot speed control signal.
  • A motor efficiency of 72,5% to achieve a minimal power consumption and maximal mechanical power output (compared to max. 50% efficiency of a normal DC motor).
  • Compact main dimensions compared to achievable output.
  • Aluminium motor housing instead 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 (1.4 Amp. at 12 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 volts sprocket drive. 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 boat and the sails should be adjusted to achieve lower rudder torques. The above table shows that the Jefa transmission 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 1.5 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 0.75 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 2 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 1.5, 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 1.5, 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 16 11*1
1¼ 13 11*1
1½ 11 11*1
1¾ 11 13
2 11 15
2¼ 11 15
2½ 11 17
2¾ 11 17
3 11 19
3½ 11 19
4 11 19
*1: This drive is as standard delivered with a 11 teeth 5/8" sprocket. Please specify when different.
Compatibility in 12 Volts. (not available in 24 volts)
Following table shows the maximum rudder torques that can be generated by the Jefa 200 Nm sprocket drive in combination with various autopilot computers. The hard over time (HO-time) states the time it takes the drive to travel the full 72 degrees of rudder travel when the speed control of the pilot is set to maximum speed.
Autopilot junction box 12 Volt version. Max. output (Amp.) Max. rudder torque (KgM)
HO-time no load   12 sec.
HO-time max. load   24 sec.
Simrad AC12 12 120
Simrad AC20 20 165
Simrad AC40 40 fully functional, but smaller autopilot is advisable (money can be saved by choosing smaller autopilot)
B&G h2000 ACP-1 25 165
B&G h2000 ACP-2 40 fully functional, but smaller autopilot is advisable (money can be saved by choosing smaller autopilot)
Nexus-Silva A-1510 (*2) 15 158
NKE gyropilot 2 RVP (*3) 25 165
Navman G-Pilot (*4) 20 165
Northstar MCU600 (*4) 20 165
Furuno Navpilot 500/511/520 20 165
Raymarine X-10 (*5)(*6) 10 120
Raymarine X-30 30 165
(*2) Please read the special installation instructions for the clutch available on our FTP server via this direct link.
(*3) Please read the special installation instructions for the clutch available on our FTP server via this direct link.
(*4) Please read the special installation instructions for the clutch available on our FTP server via this direct link.
(*5) Please read the special installation instructions for the clutch available on our FTP server via this direct link.
(*6) We strongly advise to not use the standard X10 autopilot as it delivered without rudder feedback unit. Without rudder feedback unit the autopilot is not aware of actual rudder angle. In very slow speed conditions or in conditions of a stalling rudder blade, the autopilot doesn't stop with powering the drive unit, running it against the rudder stops and still continuing to power the drive. In time the drive fuse will blow, but mechanical damage can occur in the drive unit which will not be covered under our warranty terms as we can identify this specific damage. We strongly advise only to use the X10 unit in combination with the optional rudder feedback unit or use the X30 unit.
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-WS200-12

Sprocket autopilot drive unit 200 Nm 12 Volts *1

 € 1.873

DU-TS-RFB

integrated rudder feedback unit for transmission drive*²

 € 168

CH5800

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

 € 94

CHLNK

5/8" stainless steel master link

 € 6

DU-WS-BR

Bracket for sprocket drive

€ 126

-

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.
*²: Only possible for Jefa, Garmin, Nexus and NKE autopilots (all 10 kΩ).