Inspecting Shaft Bearings Steve Zimmerman
As sailing ships yielded to steam power, the need arose for propeller shaft bearings. Builders needed a way to support a rotating shaft while minimizing wear on the shaft and maximizing longevity of the bearings. In 1852, after initial attempts using brass produced poor results, a British marine engineer named John Penn patented the first water-lubricated shaft bearing using wood. Only one species of wood has the properties needed to resist wear from a spinning metal shaft. It was called lignum vitae (Latin for wood of life because it also became known for its medicinal properties). In the early 1900s, a California miner named Charles Sherwood needed a quick fix for a failed pump at a mining site and used a rubber tube to solve the problem. The rubber worked so well that he patented the design for a rubber-sleeved bearing. More than 100 hundred years later there have been a number of improvements to this technology. One would assume that by the middle of the 20th century, nuclear submarines—ships like the SS United States— and Polar class icebreakers no longer used wood for this critical component, right? Before we answer that question, let’s look into the details of shaft bearings.
THE NEED FOR SHAFT BEARINGS
At a minimum, a propeller shaft must be supported just forward of the propeller hub. Shafts tend to fail at the point where the shaft enters the hub. The prevalence of this problem in commercial shipping has prompted multiple engineering studies and lab testing. Suffice it to say that there’s a lot going on at that spot and none of it prolongs shaft life. The shaft begins to taper at the propeller hub and the keyway begins in this area–two opportunities for weakening. In addition, dissimilar metals meet here: the bronze prop and the stainless shaft. The fit between the hub and the shaft creates a very fine opening and seawater constantly swirls around and presses into this minute gap, inviting various forms of corrosion. The forces from the propeller transfer to the shaft and then are distributed over the length of the shaft. This particular condition can be reduced by minimizing the distance between the forward end of the propeller hub and the after end of the closest bearing. ABYC standards call for a maximum distance of no greater than one shaft diameter. In other words, if you have a 2-inch shaft, the space between the forward end of the propeller hub and the after end of the closest bearing cannot exceed two inches. As the distance increases, the risk of shaft failure also increases.
In addition to mitigating failures in this critical area, we also know that long shafts need intermediate support. Although a 2-inch shaft might seem rigid, the truth is that it flexes, and if left unsupported, the shaft will whip and vibrate and create all sorts of problems. ABYC standards provide a methodology for determining the maximum distance between bearings for a given shaft configuration.
For all of the reasons described above, your boat has a least one bearing per shaft and possibly more. The bearing closest to the prop will be mounted in a separate strut, or in the stern tube at the after end of the keel. If the shaft length and diameter require additional support, there will be another bearing inside the stern tube, usually not too far from the shaft seal.
In the early 1900s, patents were filed for rubber-lined bearings, and bearings as we know them today came to the forefront. For
the record, they are not cutlass bearings–a cutlass refers to a sailor’s sword. Almost 100 years ago the trade name Cutless came
® into use, presumably to describe the fact that the rubber cut less into the shaft than wood or metal would. Duramax Marine owns the Cutless trademark and manufactures most of the shaft bearings we find in cruising powerboats today.
The basic design consists of a brass sleeve lined with nitrile. During World War II, the U.S. Navy experienced problems with natural rubber-lined bearings which overheated at high speeds and caused a variety of failures. Nitrile is a synthetic rubber, and it proved more durable and reliable under stress.
In addition to the use of synthetic rubber, these bearings protect the shaft and the liner through water lubrication. The nitrile is formed with flutes, or channels, to allow water to flow through the bearing, which in turn reduces friction and heat, and helps flush out sand and other abrasive particles. A film of water forms between the shaft and the nitrile sleeve so that the shaft rides on the water and not directly on the nitrile.
At some point the bearing will need replacement. The life expectancy of your bearing will depend on three factors: engine hours, abrasives in the water, and engine alignment. In other words, you cannot predict when it will be time for a new one. The good news is that degradation tends to be gradual.
Visual and tactile inspection determines when that time has come. First, look for signs of cracking or swelling. It should be noted that high-quality nitrile rubber bearings will not swell and will rarely get to the point where the rubber starts to look dry and cracked, but lesser brands are subject to both failures. When the rubber starts to break down in this manner, the bearing should be replaced. If the rubber looks fine, test for excess movement by taking a firm hold on the propeller and attempting to move it up and down or side to side. With a healthy bearing, you will not feel the shaft moving and bumping into the rubber. If you do feel play and a soft impact, then the bearing should be replaced.
I would like to address one area of misinformation and confusion. The test described above is effective, but certainly not technical. How much play is too much play? In an effort to quantify the answer, others have referred to published tables from ABYC. For a 1.5-inch diameter shaft, ABYC stipulates that the shaft clearance tolerances fall in the range of .004 to .009 of an inch. The Navy Technical Manual, however, states that the bearing should be renewed when the amount of play becomes .081. That’s almost 10 times higher than the ABYC standard. So what gives?
Those who cite the ABYC standard as a guide for bearing replacement misread the intention of that standard and incorrectly apply it. The table refers to acceptable tolerances for a new bearing. In other words, a brand-new bearing for our 1.5inch shaft can have up to .009 inch of play. For determining when a bearing should be replaced, the Navy Technical Manual provides the only guideline we have, and in this case the bearing should be replaced when the clearance between the bearing and the shaft exceeds .081 inch (roughly 3/32 inch). And, as it turns out, that’s about how much play is needed for you to be able to feel movement when you pull on the prop as described above. In practical terms, that remains the best test.
In some cases a neglected or misaligned bearing will wear away the shaft. In those situations the play between the new bearing and the worn shaft will exceed the minimums set forth by ABYC, and may also exceed the Navy Technical Manual’s standard for worn bearings. In these cases the bearing is fine, but the shaft diameter has been reduced by wear. Some options are available, such as fitting a sleeve onto the shaft or cladding the worn area with metal and machining, but these solutions are generally only cost-effective on shafts over 3 inches in diameter.
In addition to cracking and excess clearance, the bearing should be inspected for alignment. In many cases the shaft and the bearing meet at slightly different angles or elevations. As a result, the shaft will press more firmly on one side of the bearing and will show a gap on the opposite side.
Let’s imagine that on the after end of the strut the shaft presses against the starboard side of the bearing and shows a gap on the port side. If the same condition is true on the forward end of the strut, then the strut is parallel to the shaft, but offset horizontally. If the gaps are the opposite at the forward end, then the strut and bearing are set at a different angle from the shaft. In either case you have an alignment problem and this condition will lead to accelerated wear of the bearing at best and damage to the prop shaft at worst. In addition to bearing replacement,
the boat needs a shaft alignment repair. A misaligned bearing and shaft will often pass the pull test because the shaft will be in a bind.
The depth of the water-lubricating channels can also serve as a guide. When the channels have lost half of their original depth, replacement should be considered. Vibration and rumbling underway can also point to bearing wear, but only when the vibration gradually increases with engine hours. The sudden appearance of vibration can point to a number of possible causes, such as a dinged propeller blade or a bent shaft.
A variety of techniques can be applied to bearing removal. This topic has been covered well and will not be discussed here, but some cautionary words about installation follow.
It should be noted that the bearing must slide into the strut or stern tube with a light press fit. Pounding on the bearing with a block of wood and a hammer must be avoided. The naval brass metal sleeve will distort from the impact and bearing life will be greatly reduced. An overly tight bearing can be gradually cooled (as low as minus 20ºF) and then lightly pressed into place.
The bearing should be secured in place with cone-pointed set screws. Ideally these screws would be positioned at 4 o’clock and 8 o’clock (i.e. 60º off the centerline). This positioning creates a triangle of support—two screws set 120º apart, with the bearing contacting the housing at the apex. The brass shell of the bearing must be properly dimpled to receive these screws. The dimples enable the screws to hold the bearing without the need to tighten them to the point where they might distort the bearing or press through into the rubber. The screws should nestle into the dimples without applying pressure that might distort the bearing or score the shaft. A second set of set screws to hold the first ones in place provide an extra measure of protection. If the screws are properly installed, the relative position doesn’t matter much. The screws must be more noble galvanically than the naval brass shell to avoid losing them to corrosion.
These bearings require a flow of water for lubrication and care must be taken to avoid positioning a collar anode too close to the bearing. Water flow into and out of the bearing must remain unobstructed, and a ½-inch gap provides a good rule of thumb.
The range of tolerances described earlier provides some guidance, but the answers are certainly not black and white. An 8-knot trawler, for example, has far more tolerance for bearing wear than a boat that runs at 25 knots. A worn bearing can ruin a propeller shaft and can allow harmful vibration to pass through the drive train. It pays to keep an eye on this simple yet critical component.
What about our nuclear submarine, the SS United States, and the Polar class icebreaker mentioned earlier? What high tech material might we find in their shaft bearings? Surprisingly, all used wood, specifically lignum vitae. Even today, lignum vitae bearings have a place in critical applications such as hydroelectric power and wind generators. Nonetheless, for cruising powerboats, nitrile-lined, sleeved bearings have become the industry standard and belong on your boat.
Inset: Wooden bearings have given way to nitrile rubber, but wooden bearings can still be found in many industrial applications.
Left: A shaft bearing diagram shows water flowing into the grooves, allowing the shaft to ride on a thin film of water. This process reduces heat and friction, and the flow of water must not be obstructed on either end.
Right: Despite the remarkable size, this bearing from a commercial vessel functions on the same principles as the ones we see in our cruising boats.
Below: Inspect the bearing for uneven wear. The shaft presses against the bearing on the lower left and shows a larger gap on the upper right. The strut and the shaft have not been properly aligned, leading to accelerated bearing wear and possible shaft damage.