The appearance of extraneous noise or vibration in the bottom of the outboard motor is always an alarm for the owner of the vessel. lambIt's a sign of wear and tear or malfunction in the gearbox, and ignoring that sign can cause serious breakdowns, including jamming the transmission or destroying the body of the lower unit.
The propeller shaft is a key element in the transmission of torque from the engine to the propeller, experiencing enormous loads. Normal technical clearance It is essential for lubrication and compensation for temperature expansions, but it must be strictly controlled. In this article, we will discuss in detail how to identify critical backlash, what tools will be required to eliminate it, and why the accuracy of adjustment is more important here than in any other part of the engine.
Owners often confuse the backlash of the shaft itself with the beating of the flange or wear of slit joints. Diagnostics It requires careful attention and understanding of the design of the lower unit, and before starting to disassemble, you need to clearly determine the nature of the shaft movement when the screw is swaying.
⚠️ Attention: The propeller shaft is often accompanied by a loss of sealing of the osteoils. If you notice an oil stain on the water or an emulsion in the gearbox oil, you can not delay repairing it - the water inside will quickly destroy the gears.
Causes of Backwood in lower unit
The main reason for the free-running shaft is the natural wear of sliding or rolling bearings, depending on the design of your motor. Buses made of colored alloys or polymers wear out over time, increasing the diameter of the seat. Metal bearings They may suffer from cavitation erosion or corrosion if water gets into the gearbox.
Another factor that triggers the lash is the impact load, hitting an underwater obstacle, hitting a screw against a rock or a log, and then instantly transmitting it to the shaft. tapered They can get chipped, and the shaft can get microscopic deformation, and even if there's no visual damage, the geometry of the landing is disrupted, and the gap is widened.
The misadjustment of bearing pre-tension during the last repair often results in accelerated wear. If the wizard pulled the nuts or did not install the necessary adjustment washers, the resource of the node is reduced by many times.
- 🔩 Natural wear of sliding sleeves or rolling bodies of bearings.
- 🪨 Mechanical damage due to impacts on solid objects.
- 🌊 Corrosion and cavitation due to water entering the crankcase of the gearbox.
- 🛠️ Errors in assembly and incorrect adjustment of gaps after repair.
It's important to understand that backlash rarely occurs in isolation, and it's usually a complex problem that affects both the omentums and the slit joints. Corrosion can "eat" a seat in the body of the lower unit, which will require not just replacement of the bearing, and the weed or replacement of the entire unit.
Diagnosis: how to determine the degree of wear
The primary diagnostics are done without disassembling the engine and allows you to assess the scale of the problem, so you need to fix the dead-water and try to sway the propeller in the vertical and horizontal planes, if you feel a palpable beat or hear clicks. slug is presentBut it's important to distinguish the backshaft from the back of the screw itself.
More accurate information is given by removing the screw and swaying the tail shaft directly. Use a mounting blade or a wooden bar to avoid damaging the thread. The movements should be smooth, without jerks. If the shaft is cruising freely, as in a broken-circuit bearing, immediate intervention is required.
Pay special attention to the axial backlash. Take the tail shaft and try to move it along the axis (pull in and pull out). The permissible limits are usually fractions of a millimeter, but the specific values depend on the model of the motor. Exceeding the axial gap indicates wear of thrust bearings or fixer.
⚠️ Attention: When diagnosing, never use a metal hammer to strike a shaft or propeller. This can cause the ends to deform, making it impossible to install a new screw or requiring expensive ductwork.
Accurate measurement of gaps often requires removal of the gearbox and use of a nutrometer or micrometer. Visual inspection of bearings is also a must: look for traces of metal dying, color change (blue from overheating), or the presence of metal shavings in the lubricant. Magnetic shavings On the cone of the bearing is a sure sign of its destruction.
☑️ Luft diagnostics
Tools and training required
Quality repair is impossible without a specialized tool. The basic set includes a set of heads and keys, detachables for bearings and glands, and a dynamometer key. Without the latter, it is impossible to properly expose the preload of bearings, which is critical for the durability of the unit.
You'll also need measuring instruments, like a caliper, a micrometer, and preferably a clock-type indicator to check the shaft's beating, you'll need a degreaser and a scraper to work with the sealant and the gaskets, and make sure you have a tank for draining oil and rags.
When preparing the workplace, make sure that the sand or dirt is not allowed to enter the gearbox during assembly. Abrasive particles It will instantly disable the new bearings, and it's best to work on a clean, soft-covered workbench.
| Tool. | Appointment | Criticism |
|---|---|---|
| Dynamometer key | Strength and tightening of nuts and bolts with a given effort | High. |
| Bearing detachment | Dismantling bearings from the shaft | High. |
| Micrometer | Measurement of the diameter of the shaft and thickness of the washers | Medium |
| Beating indicator | Accurate measurement of the shaft | Medium |
| Induction heater | Heating with bearing clips for landing | Low (can be replaced) |
And the induction heater, which makes it much easier to fit new bearings, allowing the internal clip to warm evenly, but when it's not there, experienced mechanics use the oil heating method, although it's less accurate and more fire-prone. Security When working with the HS should be in the first place.
Technology of bearing replacement and bushings
The replacement process begins with the removal of the gearbox from the lower unit, and after removing the gearbox cover and removing the gear shift assembly, access to the propeller shaft is opened, and the shaft is knocked out of the body with a cutout through the end, preheating the body of the lower unit with a building hairdryer to facilitate the exit.
Old bearings are removed from the shaft by a detachable. It is important not to damage the seats on the shaft itself. If sliding bearings are used, they can be pressed or mounted with tension. In some motor models, the sleeves have a cut, which allows them to be removed without pressing, but requires accuracy.
The new parts are put in reverse order, and the rolling bearings are preheated to 80-90 degrees Celsius, which allows them to easily stand in place of the shaft without hitting the hammer. Impact landing. It is strictly prohibited, as it leads to hidden bearing defects.
Nuances of the galls installation
When installing cuff seals (oils), always lubricate the working edge with lithium lubricant. Set the oal strictly parallel to the shaft, using a mandrel along the outer diameter, so as not to skew it. The skewing of the obble will lead to instantaneous leakage and water entering the gearbox.
Pay special attention to the order of the seals. Often, the glands have the direction of rotation indicated by the arrow on the body. Installing the gland backwards will cause it to squeeze the oil out of the gearbox rather than hold it. Tightness The node depends on the correct orientation of these elements.
Adjustment of gaps and pretension
The most important step is adjustment, which is to install the correct axial clearance of the propeller shaft and ensure the pretension of the conical bearings, using calibration washers of different thicknesses that are mounted under the bearing or between the end of the shaft and the bearing.
The adjustment process often requires several attempts. First, the design washer is set up, the knot is assembled, and the backlash is measured with an indicator. If the backlash is too large, the washer is changed to a thinner (or thicker, depending on the design), if the shaft is clamped, vice versa.
To check pretension, a dynamometer key with a shaft-turning device is used, and the force required to start the shaft rotation is measured, and it must be within the limits specified in the service manual for a particular motor model. Unacceptable pretension It will cause overheating and jamming, and its absence will lead to knocking and rapid wear.
Use a "paint" (special laundry paste) to check the contact spot of the gears when adjusting the depth of engagement, which will help to determine whether the thickness of the adjusting washer under the gearbox body is correctly matched.
Once all the elements have been fully assembled and tightened, the lightness of the shaft must be checked again, rotating smoothly, without jamming or extraneous noise, and any changes in rotation after the nuts have been finally tightened indicate a misalignment or miscalculation of washers.
Assembling lower unit and checking the tightness
The final assembly includes the installation of a bearing shaft in the body of the lower unit, the installation of gaskets and the application of a sealant on the docking planes. Use only manufacturer-recommended sealants that are resistant to oils and sea water. Quality of sealing Determine the life of your repair.
Once you've assembled the gearbox, make sure you run a vacuum seal test, which is a standard procedure for detecting microcracks or loose fits in the ossels, and if the gearbox doesn't hold the vacuum, the water will eventually get in, mix with the oil, and turn into an emulsion.
Fill the gearbox with fresh transmission oil to the control hole level, and then twist the top and bottom stoppers, replacing the ossification rings first. Critical: You need to twist the plugs in a certain sequence (usually first the lower, then the upper when filling, and vice versa when draining) to avoid the formation of air traffic jams.
Well, you can't build a gearbox without vacuum testing, and even perfectly matched bearings can't save the engine if the oil seals leak water.
Once you have the lower unit on the engine, do some low-speed running tests. Listen to the transmission, check for no vibrations or leaks. Only by making sure the unit is working properly can you get to full throttle. Regularly changing the gearbox oil (once a season or every 100 motor hours) will help prolong the life of new bearings.
Frequently Asked Questions (FAQ)
Can I use a motor with a small backlash of the propeller shaft?
Operating with any noticeable backlash is not recommended. Even a small gap increases over time, leading to the splitting of the seats of the lower unit hull and the destruction of the gears of the gearbox. This also causes vibrations that destroy other engine nodes.
How often should you change the oil in the gearbox to avoid backlash?
The gearbox oil should be changed at least once a year (before winter storage) or every 100 motor hours, and regular replacement allows you to remove metal shavings and control the availability of water, which directly affects the life of the bearings.
Will more viscous oil replace worn-out bearings?
No. Using thicker oil may temporarily reduce noise, but it will not eliminate mechanical backlash; on the contrary, too thick lubrication can make it difficult for the lubrication system to work by spraying and cause the node to overheat.
How long does it take to replace the shaft bearings?
For an experienced mechanic with all the tools and parts, the job takes 4 to 8 hours. For a first-time beginner, the process can take a full-time or more, including information and preparation time.