TROU­BLESHOOTER

In­spect­ing Shaft Bear­ings Steve Zim­mer­man

Passage Maker - - Contents - BY 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.

BEAR­ING DE­SIGN

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.

RE­PLACE­MENT

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.

IN­STAL­LA­TION

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.

FI­NAL THOUGHTS

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.

Q

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.

Left: Low-qual­ity bear­ings are prone to swelling or crack­ing. The rub­ber in this bear­ing has not held up, and the bear­ing should be re­placed.

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