Trimmer gearbox – lubrication and troubleshooting

If the owner of a brushcutter uses it regularly, they may notice an increase in the gearbox temperature or the emergence of new, uncharacteristic sounds. These signs indicate that the mechanism needs lubrication. Ignoring these signs can lead to more significant problems in the future. Without proper lubrication, the gears will wear faster and may require replacement. In some cases, the part may get jammed during operation. It’s important to understand that a damaged transmission mechanism can also cause damage to other parts of the device. It is possible to repair the trimmer gearbox and lubricate the part by yourself if you know how to perform these procedures correctly and have the necessary tools and materials.

Design and principle of operation of the trimmer gearbox

A gearbox serves two main purposes: it transmits rotation from the motor shaft to the cutting unit and it can either decrease or increase the angular speed of the output shaft. When it decreases the speed, it is known as a "demultiplicator" and is commonly used on trimmers with powerful engines. On the other hand, when it increases the speed, it is called a "multiplicator" and is typically found on low-power tools. Regardless of the brand, such as "Champion" or Huter, the gearbox is always positioned in the lower part of the boom in both gasoline and electric scythes. The gearbox is composed of a pinion shaft, which transmits motion from the motor, and a gear wheel mounted on the driven shaft, which rotates the cutting block. To reduce friction during movement, rolling bearings are placed on the shaft’s bearing points. These bearings are protected by special washers to retain lubrication and prevent dirt from entering. All of these components are housed in a dural housing with a specialized grease inlet to ensure that lubrication is easy and consistent, eliminating the need for frequent disassembly. The design of the gearbox varies depending on the model and brand, such as Sterwins and "Prorab," but typically involves several gears with teeth that interlock and transfer rotation at high speeds. Due to the considerable friction generated, the gears can become quite hot.

It is crucial to use lubricant and treat the gear unit regularly to prevent overheating and breakage of the gears. Failure to do so may result in the need for replacing the damaged components.

Features of different types of gearboxes

Electric and chain saw manufacturers incorporate a variety of gearbox models into their equipment. Typically, these models vary in terms of the shape and size of the seating, which can be circular (with popular diameters of 24, 25.5, and 26mm), square, or sprocket-shaped. Another distinguishing feature between Russian, Chinese, and European gearbox brands is the different types of gears they employ. These include conical, worm, spiroid, combined, wave, planetary, and cylindrical gears. Worm gearboxes are commonly utilized in gasoline mowers.

1. A major issue is the excessive heating of the transmission unit. This can be attributed to two reasons – either the new gears have not yet been properly worn-in or the lubricant being used is inadequate, of poor quality, or altogether absent. If the problem lies with the lubricant, it should be promptly replaced with a suitable alternative. Additionally, the new gears should be gradually broken in by running them without any substantial loads, while also allowing for intermittent breaks to allow for sufficient cooling of the components.
2. The movement of the shaft is accompanied by seizing, knock, and backlash. These symptoms are indicative of bearing wear. To resolve the issue, the gearbox needs to be disassembled and a new bearing installed. The bearing damage can occur due to inadequate lubrication, lack of lubrication, excessive trimmer load, or the entry of metal objects into the seating socket, which happens when the dust cover is damaged. A special puller or a wooden block and hammer are required for removing and installing the new bearing. The hammering process should be done cautiously to avoid damaging the seating socket. In extreme cases, preheating may be necessary.
3. During mowing, the gearbox may sway or become detached from the boom. This indicates damage to the housing, requiring the installation of a new part. For minor defects, tightening the broken point with a metal clamp may suffice.
4. If the gear does not remain attached to the rod, it signifies that the joint has become worn. Consider replacing the rod with a fresh one or secure the joint with tape if it is only partially damaged.
5. The blade on the shaft may fail to rotate or cease moving when subjected to increased pressure. This issue typically arises from wear on one or more of the gears or teeth. To rectify the problem, it is necessary to replace the worn components with new ones.

The majority of gearbox malfunctions can be resolved by the trimmer owner themselves. It is simply a matter of having the appropriate tools and spare parts (ideally original components designed for the specific make and model of the brushcutter) and knowing how to disassemble the gearbox in order to replace the damaged element with a new one.

There is no need to disassemble the gearbox in order to lubricate the gear train, as there is a designated hole in the upper part of the housing specifically for this purpose. It is crucial to carefully lubricate not only the gears, but also the bearing unit. If the sealed walls of the unit wear out, the lubricant can escape from its proper position and cause vibration in the equipment. This issue is commonly observed in Chinese trimmers. To resolve this problem, it is recommended to replace the original bearings with high-quality ones. Additionally, it is important to lubricate the flywheel, as it tends to accumulate grass fragments during operation, which create extra resistance during mowing.

It is important to lubricate the starter coil dog, as it makes contact with the grooves on the flywheel during start-up. This can lead to wear and the development of mechanochemical corrosion due to the different materials used for the flywheel (aluminum) and the dog (steel).

In order to lubricate the starter coil dog, solid grease or oil is applied through a hole that is protected by a screw. The following steps should be followed:

1. Thoroughly clean the gearbox housing, especially around the plug, to prevent any dirt or debris from entering the unit.
2. Use a special device that comes with the brushcutter to remove the plug. You can refer to the image below for reference.

Pro-tip: If you need to disassemble the gearbox, make sure to clean off the old grease from the housing, shaft, and gears before applying new lubricant. It is recommended to lubricate the parts individually when they are taken apart.

It is essential to lubricate the gearbox on an annual basis. For heavy use, this maintenance procedure should be performed every 20 hours of use. If there are any signs of malfunction, it is crucial to address the lubrication points promptly.

Numerous companies in the garden tool industry, such as Stihl and Husqvarna, manufacture their own grease specifically for their products. It is highly recommended to use these types of greases as they effectively minimize wear and tear on the equipment. If the original lubricant is not accessible for the trimmer, several factors should be taken into account when selecting an alternative:

• The quality of the brand’s products that produce the lubricant;
• The viscosity of the composition, which is influenced by the shaft torque;
• The adhesion to the surface of the components of the device;
• The consumption of the lubricant, which should consider the air temperature, type, and volume of work to be performed.

Here’s a helpful suggestion! In cases where selecting a lubricant becomes challenging, it is advisable to opt for a versatile formula. Such compositions typically incorporate various additional elements that safeguard the gearbox and enhance its longevity.

When it comes to consistency, greases can exist in liquid and semi-liquid forms, containing a variety of additives, as well as plastic and solid states. Plastic greases are the most prevalent, while solid greases maintain their original state. The most popular compositions are:

• Graphite greases, which reduce friction and improve gear movement
• Lithium compositions with high anti-corrosion properties and additives that decrease metal wear
• Universal substances containing mineral oils and additives that enhance gearbox performance

Some commonly used lubricants include Litol-24, Champion EP-0, Azmol 158, and the lubricant produced by the company "Stihl".

Disassembly and replacement of the gearbox

Interchangeability of gearboxes is rarely possible due to the various gear types, seat shapes, and sizes. That’s why it’s important to consider the model of the old unit and the brand of the trimmer when purchasing a new part. If you need to use a non-original device, you should keep in mind the following details:

• The diameter of the boom is usually 24.5 or 26 mm.
• The seat shape and diameter are typically 8 mm.
• The way the protection is fixed.

If the specified criteria are identical, the previous transmission can be substituted with an appropriate aftermarket component. To substitute the transmission, it is essential to detach the device from the crane and take it apart. To accomplish this, the tie bolts located beneath the structure need to be loosened and a pair of spiral rings must be extracted using rounded pliers. Following that, the bearings on the driven and driving shafts should be removed using a specialized puller. Finally, to reconstruct the device, the removed components should be reinstalled in the opposite sequence.

Here’s a useful tip! If you don’t have a puller tool, you can gently tap the bearings out. Use a hammer and a wooden block for this task. However, be careful with your strikes to avoid damaging the seating area. In case the parts are stuck and won’t turn, you can heat them up with a construction hair dryer. The recommended temperature for heating is 600°C.

Putting the gearbox together is less complicated than taking it apart. You can easily position the structure’s components by lightly hitting them with a hammer. Once the device is fully assembled, make sure to check if the driven shaft can rotate freely when prompted by hand motion.

Breakage prevention

In order to prevent breakdowns and costly repairs, it is advisable to adhere to a set of simple guidelines for operating the trimmer and maintaining the gearbox.

1. After purchasing the garden tool, it is recommended to lubricate it immediately before using. Typically, the manufacturer (such as Interskol or Huter) only applies a small amount of lubricant, which is only enough for initial use.
2. At the end of the season or before long-term storage of the trimmer, it is necessary to disassemble the gearbox, inspect the condition of the bearings and gears, and lubricate the unit.
3. Grease should be added to the tool after it has been used for twenty to forty hours. If subjected to heavy loads, this time frame is reduced to twenty hours.
4. It is important to replace the grease with fresh grease after repairing the construction or before the start of the season.
5. If the gearbox gets too hot or makes excessive noise, it is advisable to check the amount of grease. If there is enough of it, it is recommended to completely replace it with a new one.

Therefore, it is possible to perform repairs and apply lubrication to the brushcutter gearbox on your own at home. It is not challenging to add lubricant, as a detailed explanation of the process and recommendations for selecting a suitable substance can be found in the tool’s instruction manual or in numerous online videos. The majority of gearbox malfunctions are standard for trimmers of various brands, and they can be easily rectified by following the correct procedure. By adhering to simple rules of operation and carrying out timely maintenance on the trimmer gearbox, it is possible to prevent these issues from occurring.

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