ZEXEL RHD6-MC 105856-5580 Marine Hydraulic Governor

The ZEXEL RHD6-MC, part number 105856-5580, is a high-performance hydraulic speed governor designed to regulate the speed and fuel-control operation of marine diesel engines, auxiliary engines, generator engines and other industrial power units.

The photographed governor is marked Made in Japan and has a governing speed range of 600–2000 rpm. Its nameplate specifies SAE 20W-40 oil, with an initial oil-change interval after 1,000 operating hours and subsequent changes every 3,000 hours.

 

 

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ZEXEL RHD6-MC hydraulic governor, part number 105856-5580, with speed-adjusting mechanism and connecting cable.

Product Information

Product Introduction

A marine engine governor controls engine speed by continuously adjusting the fuel-control mechanism according to changes in engine load.

When electrical load, propeller load or auxiliary machinery load changes, the engine speed naturally tends to increase or decrease. The ZEXEL RHD6-MC senses this speed variation and changes the governor output position to increase or reduce fuel delivery.

The governor therefore helps the engine:

• Maintain the selected operating speed
• Respond to sudden load changes
• Prevent excessive speed fluctuation
• Share load correctly when properly adjusted
• Maintain stable generator frequency
• Reduce hunting and unstable operation
• Support safe starting, running and shutdown

Exact Model and Nameplate Identification

The attached product photograph confirms the following information:

• Model: RHD6-MC
• Part Number: 105856-5580
• Governor Speed: 600–2000 rpm
• Oil: SAE 20W-40
• Manufacturer: ZEXEL
• Manufacturing Origin: Made in Japan
• Oil change: First at 1,000 hours and then every 3,000 hours
• Speed droop adjustment: From 0 to nearly 10%

ZEXEL RHD6-MC 105856-5580 governor nameplate

Close-up of the original nameplate confirming ZEXEL model RHD6-MC, part number 105856-5580, governing speed and lubrication requirements.

The serial number is not included as an SEO keyword because it identifies one individual unit rather than the complete product series.

What Is a ZEXEL RHD6-MC Governor?

The RHD6-MC is a mechanical-hydraulic variable-speed governor. It combines centrifugal speed sensing with hydraulic amplification to produce sufficient output force for operating the engine’s fuel-control linkage.

Inside the governor, flyweights respond to changes in rotational speed. Their movement acts through the internal control mechanism and hydraulic pilot valve. Pressurized oil then moves the power piston and governor output shaft to change the engine’s fuel setting.

This arrangement allows a relatively small change in engine speed to produce a controlled and powerful movement at the fuel-control linkage.

How the RHD6-MC Governor Works

The basic operating sequence is:

1. The governor drive rotates with the engine.
2. Internal flyweights respond to rotational speed.
3. The speeder spring establishes the required operating-speed setting.
4. Changes in engine speed alter the balance between flyweight force and spring force.
5. The pilot valve controls hydraulic oil flow.
6. The power piston moves in response to hydraulic pressure.
7. The terminal shaft changes the engine fuel-rack position.
8. The compensating mechanism stabilizes the governor response.
9. The mechanism returns to equilibrium when the selected speed is restored.

When Engine Load Increases

Engine speed initially decreases. The governor responds by moving the fuel-control mechanism toward increased fuel delivery until the engine returns to the selected speed.

When Engine Load Decreases

Engine speed initially increases. The governor reduces fuel delivery until the required operating speed is restored.

During Engine Shutdown

The shutdown mechanism moves the fuel-control linkage toward the minimum-fuel or no-fuel position, depending on the engine and linkage arrangement.

ZEXEL RHD6-MC upper speed-adjusting and control mechanism

Upper control section of the ZEXEL RHD6-MC showing the speed-adjusting mechanism, slow/fast direction indicator and governor housing.

Main Features

• High-performance variable-speed regulation
• Mechanical-hydraulic operating principle
• Governing range of 600–2000 rpm
• Adjustable speed setting
• Adjustable speed droop
• Hydraulic power amplification
• Compensation mechanism for stable operation
• Suitable for marine and industrial diesel engines
• Applicable to auxiliary and generator engines
• Robust cast-metal construction
• Mechanical fuel-control output
• Remote speed-adjusting capability, depending on configuration
• Lubricated internal moving components
• Serviceable internal mechanical and hydraulic parts
• Proven design used on many older and existing marine engines
• Made in Japan

Verified Technical Specifications

The specification and linkage settings must be confirmed against the engine maker’s manual before installation.

Speed Droop Adjustment

The nameplate states that speed droop can be adjusted from 0 to nearly 10%.

Speed droop allows the engine speed to reduce in a controlled manner as load increases. Correct droop adjustment is particularly important when generator engines are operating in parallel.

An incorrect setting may cause:

• Poor load sharing
• Unstable generator frequency
• One generator taking excessive load
• Hunting between parallel generators
• Excessive speed reduction under load
• Difficulty transferring electrical load

Speed-droop adjustment should be performed only by a qualified governor or marine-engine technician using the correct engine and generator test procedure.

Main Components

The RHD6 governor family typically incorporates:

• Governor drive shaft
• Flyweight assembly
• Speeder spring
• Speed-adjusting mechanism
• Speed-droop mechanism
• Pilot valve
• Hydraulic oil pump or gear pump
• Power piston
• Terminal or output shaft
• Compensating piston
• Compensation needle valve
• Internal linkage
• Control lever
• Shutdown mechanism
• Governor housing
• Oil reservoir
• Bearings and bushes
• Oil seals and O-rings
• Gaskets
• External fuel-control linkage
• Remote speed-adjusting device, where fitted
• Electrical cable and motor arrangement, depending on configuration

The exact internal component numbers can vary according to the complete part number and production revision.

Typical RHD6 governor construction showing the internal speed-sensing, hydraulic-control and output mechanisms. The exact construction should be verified against the relevant ZEXEL service manual.
Technical image source: Diesel Kiki RHD6 technical reference

Marine Applications

The ZEXEL RHD6-MC may be used for speed control on:

• Marine auxiliary diesel engines
• Shipboard generator engines
• Diesel generator sets
• Main propulsion engines, where specified
• Emergency generator engines
• Industrial power engines
• Stationary diesel engines
• Pump-driving engines
• Compressor-driving engines
• Other constant- or variable-speed diesel applications

Typical vessel types include:

• Cargo ships
• Tankers
• Bulk carriers
• Container ships
• Offshore support vessels
• Passenger ships
• Ferries
• Fishing vessels
• Tugboats
• Workboats
• Research and government vessels

Compatibility must be determined from the exact engine model, original governor number, drive arrangement, output linkage and control requirements.

Common Problems and Troubleshooting

1. Engine Speed Hunting

Hunting occurs when engine speed repeatedly increases and decreases instead of remaining stable.

Possible causes include:

• Incorrect compensation adjustment
• Air trapped in the governor oil
• Contaminated or unsuitable oil
• Low governor oil level
• Sticky pilot valve
• Worn internal components
• Excessive clearance in the engine fuel linkage
• Incorrect speed-droop setting
• Uneven engine combustion
• Fuel-pump or injector problems

The engine fuel system should also be checked before concluding that the governor alone is responsible.

2. Engine Speed Does Not Increase

Possible causes include:

• Speed-setting mechanism fault
• Remote adjusting motor failure
• Damaged cable or electrical connection
• Restricted terminal-shaft movement
• Seized external fuel linkage
• Low internal oil pressure
• Pilot-valve sticking
• Incorrect linkage adjustment
• Internal wear or damage

3. Engine Speed Does Not Decrease

Check for:

• Sticking fuel rack
• Binding external linkage
• Incorrect stop adjustment
• Governor output shaft restriction
• Speed-setting mechanism jam
• Internal pilot-valve fault
• Contaminated governor oil
• Incorrect assembly after overhaul

4. Slow Response to Load Changes

Potential causes include:

• Incorrect oil viscosity
• Low oil level
• Contaminated oil
• Air in the hydraulic circuit
• Restricted internal oil passages
• Worn oil pump
• Incorrect compensation adjustment
• Power-piston leakage
• Excessive mechanical linkage friction

5. Governor Oil Leakage

Oil leakage may occur from:

• Worn shaft seals
• Damaged O-rings
• Hardened gaskets
• Loose covers or plugs
• Cracked housing
• Excessive internal pressure
• Incorrect oil filling
• Damaged sealing surfaces

The leakage source must be identified before seals or gaskets are replaced.

6. Engine Cannot Maintain Rated Speed

Inspect:

• Governor drive speed
• Fuel-rack movement
• Engine fuel supply
• Governor oil pressure
• Speeder-spring condition
• Pilot-valve movement
• Terminal-shaft travel
• External linkage adjustment
• Engine load
• Cylinder combustion performance

7. Unstable Generator Frequency

Possible causes include:

• Governor hunting
• Incorrect droop adjustment
• Faulty remote speed-setting motor
• Loose or binding fuel linkage
• Sudden load variation
• Poor generator load sharing
• Worn governor components
• Engine fuel-system problems

8. Governor Produces Unusual Noise

Unusual mechanical noise may indicate:

• Low oil level
• Worn bearings
• Damaged gear pump
• Loose internal component
• Drive misalignment
• Excessive mechanical clearance
• Contaminated or incorrect oil

Stop and inspect the governor if severe noise, vibration or overheating occurs.

Common Alarms and Fault Indications

The ZEXEL RHD6-MC is primarily a mechanical-hydraulic governor and does not normally have a digital display or built-in numerical alarm-code system.

Any alarm may instead be generated by the engine’s external monitoring and protection system, such as:

• Engine overspeed
• Engine underspeed
• Low lubricating-oil pressure
• Generator frequency high
• Generator frequency low
• Load-sharing failure
• Start failure
• Emergency shutdown

No numerical alarm codes should be assigned to the RHD6-MC itself unless they are documented in the connected engine-control system.

Maintenance Tips

• Check the governor oil level regularly.
• Use the specified SAE 20W-40 oil unless the applicable service instruction states otherwise.
• Change the oil after the first 1,000 operating hours.
• Continue oil changes approximately every 3,000 operating hours.
• Keep the oil and filling equipment clean.
• Avoid mixing incompatible oil types.
• Inspect the unit for external oil leakage.
• Check the speed-adjusting mechanism.
• Inspect the terminal shaft and external linkage.
• Ensure that linkage movement is smooth and unrestricted.
• Check the cable and electrical connections where a remote motor is fitted.
• Monitor engine speed and generator frequency.
• Investigate hunting before changing internal adjustments.
• Record all original adjustment positions before disassembly.
• Use the correct overhaul procedure and calibrated test equipment.
• Bench-test the governor after major repair.
• Confirm overspeed protection independently after installation.

Governor calibration, internal adjustment and overhaul should be performed by experienced technicians. Incorrect adjustment may cause unstable engine operation, overspeed or failure to accept load.

Spare Parts & Replacement Availability

SM MARINE SERVICE™ can support the sourcing of the ZEXEL RHD6-MC 105856-5580 complete governor and associated spare parts, subject to availability.

Possible supply items include:

• Complete RHD6-MC governor
• Governor speed-adjusting motor
• Remote speed-control mechanism
• Governor repair kit
• Seal and O-ring kit
• Gasket kit
• Bearing and bush set
• Flyweight assembly
• Speeder spring
• Pilot valve
• Gear pump components
• Power piston
• Compensation components
• Speed-droop mechanism
• Terminal shaft
• Drive shaft
• Control lever
• Shutdown mechanism
• Oil seals
• Internal springs
• Internal mechanical components
• Cable and connector assembly
• Replacement housing
• Tested exchange governor

Available conditions may include:

• New
• New old stock
• Used
• Refurbished
• Reconditioned
• Overhauled
• Tested working
• Exchange basis

Important Replacement Information

A physically similar RHD6 governor should not automatically be treated as an exact replacement.

Before ordering, confirm:

1. Exact model number
2. Complete part number
3. Governor speed range
4. Engine manufacturer and model
5. Engine rated speed
6. Generator frequency, where applicable
7. Drive-shaft configuration
8. Mounting dimensions
9. Terminal-shaft direction and travel
10. Fuel-rack linkage arrangement
11. Speed-adjusting motor specification
12. Shutdown arrangement
13. Speed-droop requirements
14. Photographs of all nameplates and connections

For this unit, the primary matching references are RHD6-MC and 105856-5580.

Why Marine Engines Still Use the ZEXEL RHD6-MC

Many vessels continue using RHD6 governors because the design is robust, serviceable and suitable for long-term operation.

Key benefits include:

• Reliable mechanical-hydraulic control
• Strong output force
• Stable speed regulation
• Adjustable droop
• Repairable internal components
• No dependence on complicated software
• Compatibility with established engine installations
• Long operating life with proper maintenance
• Availability of used and reconditioned equipment
• Familiarity among experienced marine engineers

Replacing an existing governor with a different electronic control system may require major changes to the fuel linkage, engine protection, wiring, control logic and classification documentation. Consequently, sourcing the correct RHD6-MC replacement is often the most practical solution.

Frequently Asked Questions

What is the ZEXEL RHD6-MC?

The ZEXEL RHD6-MC is a mechanical-hydraulic variable-speed governor used to control diesel-engine speed and fuel delivery.

What is the part number shown in the photograph?

The photographed governor shows part number 105856-5580.

What is the speed range of this governor?

The nameplate shows a governing speed range of 600–2000 rpm.

Which oil is recommended?

The product nameplate specifies SAE 20W-40 oil.

How often should the governor oil be changed?

The nameplate instructs an initial change after approximately 1,000 hours and regular changes every approximately 3,000 hours.

What is the speed-droop adjustment range?

The nameplate states that speed droop can be adjusted from 0 to nearly 10%.

Where was the ZEXEL RHD6-MC manufactured?

The photographed unit is marked Made in Japan.

Is ZEXEL related to Diesel Kiki?

ZEXEL products are commonly associated with the former Diesel Kiki brand name. Both names may therefore appear in searches for RHD6 governors and spare parts.

What causes an RHD6-MC governor to hunt?

Possible causes include incorrect compensation, contaminated oil, trapped air, low oil level, sticking internal parts, worn components, loose fuel linkage or an engine fuel-system problem.

Can a different RHD6-MC part number replace 105856-5580?

Not without technical confirmation. Different part numbers may have different speed settings, linkage arrangements, motors, shafts or engine applications.

Can I buy a used or refurbished RHD6-MC?

Yes. SM MARINE SERVICE™ can support the sourcing of used, refurbished, reconditioned and tested RHD6-MC governors, depending on availability.

Are spare parts available for the RHD6 governor?

Complete governors, repair kits, seals, bearings, hydraulic components, speed-control parts and other internal components may be available depending on the exact part number.

Where can I buy ZEXEL RHD6-MC 105856-5580?

International buyers can contact SM MARINE SERVICE™ for availability, technical matching, condition confirmation and worldwide shipment.

Can SM MARINE SERVICE™ supply this governor worldwide?

Yes. Worldwide sourcing and shipping support can be provided to shipowners, ship managers, marine engineers, shipyards, power plants and marine spare-parts buyers.

Short Product Description

The ZEXEL RHD6-MC 105856-5580 is a Japanese-made hydraulic variable-speed governor for marine diesel engines, auxiliary engines and generator applications. It features a 600–2000 rpm governing range, adjustable speed droop and hydraulic fuel-control output. The nameplate specifies SAE 20W-40 oil, with the first oil change after 1,000 hours and subsequent changes every 3,000 hours. SM MARINE SERVICE™ can support the worldwide supply of used, refurbished, reconditioned and tested ZEXEL RHD6-MC governors and spare parts.

User Manual & Technical Resources

???? ZEXEL RHD6 and RHD10 Service Manual

The RHD6/RHD10 service documentation covers:

• Governor specifications
• Internal construction
• Operating principle
• Engine starting and stopping
• Load increase and decrease
• Speed-droop mechanism
• Compensation system
• Troubleshooting
• Disassembly and inspection
• Maintenance and adjustment

View ZEXEL RHD6 and RHD10 Service Manual Reference

???? RHD6/RHD10 Technical Manual Reference

View RHD6 and RHD10 Technical Information

These are public third-party manual references. The exact service-document revision should be confirmed before overhaul or calibration. No official free direct-download manual from the original manufacturer was verified, so an unverified manufacturer link has not been invented.

SM MARINE SERVICE™
Marine Automation & Navigation Equipment Supplier

Available Condition:
• New Equipment • Used Equipment • Refurbished Equipment • Reconditioned Equipment

Stock Location:
• Bangladesh • China

Services:
• Worldwide Shipping • Technical Support • Spare Parts Supply • Marine Equipment Sourcing

Contact Information
SM MARINE SERVICE™ Samir Market, DT Road, Pahartali, beside CDA Market, Chattogram, Bangladesh
Mobile / WhatsApp / Viber: +88 01818 929292
Email: info@smmarineservice.com
Website: www.smmarineservice.com