The reliability of your boat’s course depends on the state of the lower part of the engine, where one of the most loaded nodes is hidden. gearboxIt's the one that transmits the rotation from the crankshaft to the propeller, converting high revs into high thrust, and many outboard motorists forget about this knot until they hear an outside noise or notice a loss of speed.
Inside the hull lower unit There is a complex mechanical work that involves constant friction and impact, and understanding how this works not only avoids costly repairs, but also extends the life of the engine, and in this article we will discuss in detail the design, typical malfunctions and maintenance nuances.
Principle of operation and mechanism design
The main task of the gearbox is to reduce the speed of rotation and increase torque. The internal combustion engine produces maximum power at high revs, which would be disastrous and inefficient for the screw. Transfer ratio determines how many times the speed of rotation of the screw is less than the speed of rotation of the crankshaft.
The structure is a sealed aluminum housing, inside which are shafts and gears. gear-coupleThis is a system that allows you to move the motor to front, reverse or neutral modes, and the tightness is ensured by the glands, the condition of which is critical for the survival of the mechanism.
Most modern two-stroke and four-stroke motors use conical hypoid gear, providing smooth running and high load capacity. Lubrication of rubbing pairs is carried out by transmission oil poured inside the body, which also performs the function of heat removal.
Why is the screw rotating the other way?
In some engines (especially the Yamaha and Suzuki), the forward rotor rotates counterclockwise (when viewed from the rear), a feature of the gearbox design aimed at compensating for engine torque and improving the boat's handling.
Typical malfunctions and their symptoms
Diagnosing gearbox problems often begins long before the gearbox is disassembled, the attentive operator will notice changes in the boat's behavior or hear changes in the sound of the engine, ignoring the first signs can lead to jamming of the mechanism and the hull collapse.
One of the most common problems is that water gets inside the housing, which is caused by the wear of the osteoils or by the appearance of microcracks, and the emulsion that is formed when oil and water mix loses its lubricating properties, which leads to accelerated wear. bearing and gears.
Mechanical damage often occurs after contact with underwater obstacles. A hit on a rock or snag can displace the shafts, disrupt the engagement of gears or damage the gears. slatIn such cases, the motor may stall at high speeds or emit a characteristic metal ringing.
Oil replacement procedure in the gearbox
Regular oil change is a basic maintenance procedure that each owner must perform independently, which removes wear products and water that may have condensed inside, and the frequency of replacement depends on the intensity of operation, but is usually once a season or every 100 motor hours.
To perform the procedure, you will need a syringe pump, a new oil of the appropriate viscosity and new sealing pads for drain stoppers.
First, the top plug is pulled out, then the bottom plug, allowing the old liquid to completely leak out. If the oil is "coffee with milk", this is a sure sign of water entering and the need to repair the seals. After washing the case (if necessary), fresh oil is filled up to the level of the top plug.
☑️ Oil change checklist
Table of viscosity and oil types
Choosing the right lubricant is critical to the durability of the unit. The use of oils for car gearboxes is often unacceptable, as they may contain additives aggressive to non-ferrous metals from which the gearbox sleeves are made.
Special oils with labelling must be used GL-5 specifications of the motor manufacturers (e.g., Yamalube Gear Oil or Mercury Gear LubeThe viscosity is selected according to climatic conditions and water temperature.
| Type of oil | Temperature conditions | Application |
|---|---|---|
| SAE 90 | Above +5°C | Summer operation, warm regions |
| SAE 80W-90 | From -10°C to +35°C | All-season use |
| SAE 75W-90 | Below -15°C | Extremely low temperatures |
| GL-5 | Universal. | Standard for hypoid transmissions |
Diagnostics and repairs DIY
If you find a backlash or hear a hum, you'll need a deeper diagnosis. paddle-bearer To make a precise diagnosis, the gearbox must be removed from the engine and disassembled on the workbench.
Disassembly requires care and purity. All parts should be arranged in the order of their removal. Pay special attention to the condition of the gears: chipped, painted metal or bullying indicate the need to replace a pair of gears, not one part.
When assembly, it is important to correctly set the gaps between the gears and bearings. This is done using special calibres and adjusting washers. Puff moment The mounting bolts must meet the manufacturer's specification so as not to deform the body.
⚠️ Warning: Never start the motor with the gearbox removed or without the screw on the shaft. Lack of water resistance can lead to the swirling of the ossels and the release of oil, as well as to overheating of the cooling pump, if it has not been previously lubricated.
Conservation and storage in the off-season
Proper preparation for winter avoids starting problems and corrosion at the start of the next season, and if the engine is kept upright, the gearbox oil may not be drained, but it should be checked in the spring.
In the case of horizontal storage or long-distance transport, it is recommended to ensure that the plugs are leaky. Corrosion of external elements also poses a threat: magnesium anodes installed on the gearbox protect the aluminum housing from electrochemical corrosion.
Check the condition of the anodes regularly, if they have dissolved by more than 50%, they must be replaced, otherwise the gearbox or propeller will corrosion, leading to costly repairs.
How often should you change the oil in the gearbox?
The optimum frequency is once a year before the start of the season or every 100 motor hours. If the engine is new, the first replacement is better done after running (10-20 hours) to remove metal shavings.
What to do if an emulsion appears in the oil?
The milk color of the oil indicates water. You need to find the cause (wear of the glands, crack, loosely twisted cork), eliminate it, wash the gearbox and replace the oil. Operating with the emulsion will lead to the destruction of the bearings.
Can I use solidol or litol?
Absolutely not. These lubricants are not designed for high-speed hypoid gears, and they don't have the necessary anti-wear properties, and they're guaranteed to cause the gearbox to fail.
Why is the gearbox buzzing at high speeds?
The hum can be caused by wear on bearings, improper engagement of gears after repair, or insufficient oil levels, and sound can be transmitted from the engine through lower unit, so it is important to conduct acoustic diagnostics.
How to check the tightness of the gearbox at home?
There is a method of checking the pressure: a small excess pressure (about 0.5-0.7 atm) is applied to the gearbox through the oil hole by air, the knot is immersed in water or the joints are lubricated with soap solution, the appearance of bubbles will indicate the location of the leak.