BEST PRACTICES Shaft Couplings

The marriage of engine to propeller shaft is the most essential link in the drivetrain. Get it wrong at your peril.

Text and photographs by hile relatively simple in design (Note: The bushing is vulnerable to Steve D’Antonio Wand construction, propeller shaft damage when not mated to the trans­ couplings are the sole mechanical tie in mission.) The two round flanges are the drivetrain between the propeller secured to each other with multiple shaft and the transmission and engine. common fasteners running through To fulfill their essential role in a boat’s them. While these are the basics, there propulsion system, they must be prop­ are variations. erly selected and installed. The design of a coupling is simple. Coupling Types It’s a wide, flat flange mounted on the For vessels up to about 75' (22.9m), end of a driveshaft that interfaces with there are three common styles. The a mating face on the transmission’s out­ solid, or straight-bore, coupling relies put coupling. To keep the two faces on a bore the same diameter from end Above—In this split shaft coupling, grade 8 machine screws fasten the centered on each other, the shaft cou­ to end, and engages the propeller shaft pinch and the flange. The flange pling incorporates a male pilot bush­ with a light interference fit. It is typi­ cap screws are too short; they should ing, which stands proud of the face and cally pushed onto the propeller shaft protrude slightly beyond the nut. Note registers with a corresponding female using a mallet, or a maul and block of the coupling is designed to be pinned. recess in the transmission coupling. wood. A coupling should never be

56 Professional BoatBuilder struck directly with a metallic ham­ mer or maul, because doing so could damage the finely machined coupling face and pilot bushing. A small ham­ mer mark can create 0.01"–0.02" (0.25mm–0.51mm) of raised material on the face, causing the shaft to have run-out (deviation from the transmis­ sion’s center of rotation), which will probably lead to vibration. Straight-bore couplings are com­ mon in low-horsepower applications, including sail auxiliaries. While they work, they lack the shaft holding power required for larger engines and props, which can place significant strain on the shaft-to-coupling inter­ face. Getting the interference fit between shaft and coupling bore just right is a challenge. If it’s too tight, from the boat) by driving a pair of Above—Pilot bushings are critical to driving the coupling onto the shaft steel wedges or cold chisels into the centering the shaft on the transmission output coupling. This coupling’s pilot may be difficult if not impossible, gap between the two split halves. bushing has been damaged by impacts especially in confined enginerooms Beyond this, I see no other benefit to in two locations. Below—The primary with no space to swing a mallet. If it’s using a split coupling, and would benefits of a split coupling are its easy too loose, the shaft may begin to work argue that rather than make it easy to removal and, when compared to tapered in the coupling with each shift evolu­ remove, the primary mission of a cou­ couplings, the reduced cost of the shaft tion, which in turn will gall the shaft pling is to reliably secure the shaft. machining required. key, allowing for more and more And, because it is split, it moves or movement with continued wear. It distorts each time the shaft is inserted often ends when the key and set screws or removed. When that happens, the or roll pins shear off, allowing the shaft coupling face may not remain truly and prop to slide out of the shaftlog to perpendicular to the shaft. That’s strike, and possibly jam, the rudder. because it is easy to make an instal­ The worst-case outcome is that it lation mistake—not torqueing doesn’t jam, and the shaft and prop pinch bolts in proper sequence, slide out of the stuffing box altogether, or pinching against a set screw, leaving a shaft-sized hole with a geyser which leads to the coupler being of seawater into the bilge, likely over­ off perpendicular, resulting in a whelming the pumps. nasty vibration. So, unless you know Split couplings are easily identified you’ll want to remove your shaft by their bifurcated design. While this regularly, there’s little to recom­ coupling is made up of one part, the mend this type. aft plate is divided into two sections, The third common option is a which are clamped against the shaft by taper coupling. Its bore is cone- two or more machine screws (these shaped in precisely the same should be SAE/Society of Automotive manner as a propeller hub bore, Engineers grade 8). With this arrange­ and it engages an identical male ment the coupling can actually apply cone machined in the forward clamping action to the shaft. Its main end of the prop shaft. I prefer to advantage over other coupling types is install propellers on a taper engage­ that it enables easy removal from the ment because it’s extremely reliable: shaft (and thus easy shaft removal it can never be too tight or too loose;

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Left—Tapered couplings offer several advantages over straight and split models. The interface between the taper shaft and the bore makes for an exceptionally tight fit. In some cases where locking nuts have loosened, the shaft and coupling must still be separated with a hydraulic puller. Right—This tapered coupling has the added security of set screws to retain the locking nut. it never compromises the perpendicu­ by grinding away small elevations this approach, typically found on shafts larity of the shaft and propeller when or ridges in the interface. It should 4" (102mm) and larger in diameter, is installed correctly; and once engaged be carried out the first time a tapered ease of access. The nut requires a large it is highly unlikely to shift or separate. shaft and coupling are mated, or when 1" (25mm) breaker bar and a 3" or 4" Its primary weakness is the potential it’s unknown whether they’ve ever been (76mm or 102mm) socket for removal for key binding during installation. In lapped. Thereafter it’s unnecessary. and installation, room for which some­ short, this is an excellent means of Another tapered-shaft-retention times doesn’t exist. The keeper-plate securing the shaft to the coupling, so method applies what’s referred to as bolts, on the other hand, can use a let’s look at some details. a “three-bolt keeper plate” in place standard ½" or ¾" (13mm or 19mm) The shaft end fitted to the taper cou­ of the large single locking nut. This ratchet drive, a small Allen socket, and pling is threaded and retained by a alternative hardware relies on three far less torque. large nut housed in a recess in the cou­ recessed Allen-head machine screws Other than the set screws, none of pling face. As on a propeller, the nut is threaded into holes parallel to the these retention components are visible commonly augmented by a cotter pin shaft centerline, located in the cou­ after the coupling is bolted to the trans­ (sometimes heavy seizing wire) or is pling end of the shaft. A metal plate mission. I have separated many taper replaced with a nylon locking nut. In located under the heads of the fasten­ shaft couplings from their transmission some cases, set screws installed in the ers acts as a retainer. The advantage to flanges, only to find the retaining nut coupling, perpendicular to the shaft and close to the flange end, are tight­ ened against the shaft nut flats, thereby locking it in place. Much like tapered propeller instal­ lations, tapered couplings benefit from lapping, wherein the coupling and shaft tapers are custom fit to fine tolerances with the help of valve- grinding compound. The goal is to maximize contact between the surfaces

Reverse-taper couplings like this one are often used with V-drive transmissions wherein the shaft passes through a “port” in the transmission case.

58 Professional BoatBuilder Best Practices: Shaft Couplings

loose but the shaft still firmly engaged in the coupling bore—evidence of the taper design’s effectiveness. Another variation on this coupling is the reverse taper. In a conventional taper the large end of the bore is far­ thest from the coupling flange. The shaft is inserted onto this end. In a reverse taper, the large end of the bore is at the flange end. Reverse tapers are most often used in -V drive applica­ tions, where the shaft passes through the transmission. In practice it works the same as a conventional taper; however, the retaining nut and cotter pin are exposed after assembly is complete. On occasion I’ve recommended that clients specify a tapered coupling for a new or replacement shaft, only to Above—This red nonmetallic have them report back that the boat­ coupling insert is designed to builder or shaft shop doesn’t recom­ shatter if the prop strikes a mend this approach. When quizzed, large object. Insert surfaces lack they often have no explanation, other the precision required for accu- than being told, “A split coupling is rate alignment measurements. better.” Invariably, when I explore the Before an alignment, the insert issue by speaking directly with the must be removed and the coupling faces brought together. shaft shop, I learn that they do not If shaft length or the insertion possess the machinery necessary to of line cutters won’t permit that, make a tapered shaft and coupling. In a precision spacer like the one practice, many smaller shaft shops shown at left can be installed to purchase shafts with the prop-shaft- perform the alignment. end taper already cut, and then they simply cut the shaft to length and machine the keyway into the coupling movement into heat. Consequently, in in vibration. I have seen cases where end. This is a convenience and cost- all cases they eventually wear out. The inserts, as a result of their imprecise saving measure for the shop, but it moment when this occurs can be either plastic faces, exacerbate misalignment offers no benefit to the end-user. For insidious, when they simply stop absorb­ in otherwise well-installed and -aligned all but the smallest shafts, it pays to ing movement, giving the user no obvi­ drivelines. Also, they complicate align­ insist on tapered couplings. ous indication, or catastrophic, when ment procedures, which, added to their they fail completely. tolerance of misalignment, can lead to Flexible Couplings and Some flexible-coupling manufac­ acceptance of lower standards for the Inserts turers claim their products reduce final installation. Among the most effec­ Discussions of the marriage of shafts vibration in addition to absorbing tive means of reducing drivetrain noise to transmissions would be incomplete misalignment, while other manufac­ and vibrations are combination thrust without mentioning flexible couplings. turers claim theirs shear or shatter in bearings and universal or CV joints. (For These come in two primary varieties: the event the prop strikes a log, rock, more on thrust bearings, see PBB No. those that are an integral part of the cou­ etc., thereby preventing or minimizing 120, page 42.) pling itself, and those that are inserted damage to the transmission. (The only For a coupling alignment, the non­ between the two coupling halves. True times I’ve seen them shear is as a result metallic inserts must be removed and flexible couplings are designed to absorb of stresses induced by misalignment, the coupling faces either engaged some limited degree of misalignment not from striking an object.) directly, or, if that’s not possible, a pre­ between the shaft and engine/transmis­ In my experience, there’s no substi­ cision-machined metal insert installed sion. They do so by including a flexible tute for proper alignment, and I have (these are available from Spurs, the component in the driveline, usually rub­ never known a flexible coupling or manufacturer of line and net cutters), ber or polyurethane, which converts insert to make a noticeable difference the alignment checked and corrected,

June/July 2019 59 Best Practices: Shaft Couplings

Left—Set screw tips should be either cupped or pointed, and hardened. Ordinary stainless steel fasteners, shown here, should not take the place of purpose-built screws. On the plus side, these set screws are secured with locking nuts and seizing wire. Above—Scallops cut into this shaft indicate that it’s designed for use with a split coupling. Pinch bolts interface with the machined dimples to improve shaft-to-coupling engagement.

horsepower applications screws (usually installed 90° apart); a require nothing more than clevis pin (a straight, solid, round pin SAE grade 5, but if you are retained by friction alone); a roll pin (a purchasing the fasteners straight, round, hollow pin split and for this application, it’s sprung—the spring tension holds it in worth paying a little more place); or a taper pin (a cone-shaped for the added strength. pin threaded at the small end, inserted and only then should the nonmetallic (For more on fastener selections and into a cone-shaped bore drilled through coupling insert be reinstalled. grades, see “Nuts. Bolts. Screws.” in the shaft and coupling, and secured In the end, I question whether all this Professional BoatBuilder No. 118.) with a nut). For split couplings, another time, trouble, expense, and potential These fasteners should be mated to retention method may be used, one sacrifice in reliability are worthwhile. split lock washers, or nylon-insert lock where scallops or half rounds are cut Other than the precision metallic nuts when installed, and neither should into the shaft perpendicular to its cen­ insert, I’m leery of installing any com­ be reused in a reassembly. For espe­ terline, which are then engaged by the ponent into the drivetrain that isn’t cially hard-working applications, where coupling’s clamp bolts. absolutely necessary, as it simply pre­ frequent shifting and throttle work is Unless clevis and taper pins and their sents another potential point of failure. anticipated (passenger and yacht club matching bores are precisely machined launches, for example), consider using and the pins properly installed, they Fasteners, Set Screws, and cam-style Nord-Lock washers, which can cause problems. I’ve removed clevis Clevis, Taper, and Roll Pins are significantly more effective than and taper pin-equipped couplings only The grade of fasteners connecting conventional lock washers or nuts to have the pins fall out in two or three the two couplings should be a min­ and virtually fracture-proof. Wherever pieces, or to have them seize in the cou­ imum of SAE grade 5, identified by clearance permits, use a torque wrench pling and shaft, requiring tedious drill­ three hash marks on the head in a Y for installation of all coupling fasteners. ing, driving out with a drift, or cutting pattern (for metric fasteners the head When the couplings are perma­ the shaft. Note that ordinary hex-head includes the numbers 8.8) and ideally nently connected, they and the secur­ machine screws should never be used SAE grade 8, with six evenly spaced ing hardware should be coated with in this through-shaft application, as hash marks on the head (and the num­ a rust preventative such as CRC’s they lack the necessary precision in siz­ bers 10.9 for the metric equivalent), all Heavy-Duty Marine Corrosion Inhib­ ing and the overall strength required. mild steel, fine thread. While stainless itor, or painted. Fasteners available in Especially after wrestling to disas­ steel is desirable elsewhere aboard, in a plated finish are desirable as it will semble a corroded shaft coupling, it’s this location corrosion resistance takes further inhibit corrosion. tempting to apply anti-seizing com­ a backseat to tensile strength, and for In all the above coupling arrange­ pounds or grease to facilitate removal, like dimensions, mild steel is simply ments, save the taper variety, one or but these should never be used on taper stronger than stainless steel, and SAE more of several retaining methods are fittings, including the shaft taper itself grade 8 is stronger than the more used to further secure the shaft to the and taper pins. The interference of common SAE grade 5. Many lower coupling, including: one or two set viscous material can prevent proper

60 Professional BoatBuilder Best Practices: Shaft Couplings

Far left—Grease on this coupling and key was applied to ease instal- lation but will actually prevent full engagement between the shaft and coupling, as it cannot be com- pressed in the confined coupling bores. Shafts, keys, and couplings may be lightly oiled to prevent binding and ease installation. Left—This brass taper coupling relies on set screws to secure the shaft end nut. Note that the seiz- ing wire is correctly tensioned to rotate the screws clockwise.

engagement of the tapered shaft and be installed with a thread-locking com­ typically interfaced or “keyed” to the the coupling surfaces. The same prohi­ pound. Take the belt-and-suspenders shaft using a rectangular section of bition on anti-seize is true for other approach and use a locking compound precision-machined stainless steel coupling fasteners. A coating of light oil in tandem with seizing wire; in addi­ stock known as a key set in a corre­ may be applied to prevent binding and tion to preventing loosening, the com­ sponding keyway milled longitudi­ inhibit corrosion on all coupling com­ pound can also serve as a corrosion nally into the shaft and coupling. The ponents including the coupling bore, inhibitor by preventing water migra­ keyway should be radiused, meaning shaft surface that’s inserted into the tion into the threads. the interface between its vertical bore, key, keyway, and coupling faces. For through-shaft applications, “walls” and “floor” should be slightly Do not use it on threaded components ordinary machine screws should never rounded, and the aft termination of where you intend to apply thread-lock­ be used in place of a hardened set the shaft keyway should be ramped. ing compounds. Remember, the torque screw, as they lack the necessary ten­ These features minimize the formation rating of a fastener whose threads have sile strength and proper tip design. of stress risers and cracking. been oiled is lower than one whose The style of set screw tip plays an The key-to-keyway fit is critically threads are dry. Charts for this are important role in securing the shaft. If important for proper engagement readily available online. the tip is pointed, as some set screws between shaft and coupling: too loose Set screws come in two varieties: are, then the shaft must be drilled with and movement may occur; too tight and square head and recessed Allen- a matching pointed dimple of the binding may occur between key, shaft, socket-drive heads. (The latter is same angle, into which the screw point and coupling. If lightly oiled and sometimes referred to as a grub screw.) fits snugly. If, on the other hand, the installed, the key should move in the Ideally, square-head set screws should set screw is cupped, then a dimple is keyway with fingertip effort; however, it include a hole through the squared not necessary and should not be used. should not be possible to rock the key section, which accommodates the Like propellers, all couplings are or produce any clicking sound. When added security of stainless or Monel seizing wire to keep the screw from backing out. There is an art to installing seizing wire for this purpose—it must be neat and tight. And always tension the fastener in the clockwise direc­ tion. Wire diameter should be selected for the largest size that will fit through the available hole, and because square-head set screws are hardened, those with­ out holes cannot be easily drilled after the fact. If you wish to seize Left—A ramped and radiused keyway is more resistant to stress-induced fractures, while the your set screws, order them with knurling improves the interference fit on this small sail auxiliary straight-bore coupling. holes predrilled. Those that can­ Right—Keys should fit snugly into a keyway and, when lightly oiled, should slide and be easily not accommodate seizing should removed. Unlike this one, they should not rock or clack when moved from side to side.

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Fitting and Facing If left uncoated in a damp bilge, mild While shafts and couplings are steel couplings will rust. Couplings and precision-made and interchangeable, fasteners should be painted or coated once mated they represent a custom with a corrosion inhibitor that will not assembly. That’s why some element of be flung off a spinning shaft custom matching is required for shafts and couplings mated for the first time (if either or both parts are new). It’s a their couplings. No surprise, as process known as fitting and facing, most couplings are made of ordi­ which should be carried out by a nary ductile iron (though some machine shop that specializes in prop are brass), which is prone to rust. shaft work. When the shaft is seized in a rusty The coupling face and pilot bushing coupling, the labor to remove it should be carefully inspected to ensure sometimes makes cutting, and thereby that they are free of dents, gouges, rust, destroying, the shaft a less expensive or other irregularities. (Even new cou­ alternative. This is especially true for plings should be inspected, as they can the three components—shaft, coupling, smaller diameter (less than 2"/51mm), be damaged while in storage or during and key—are engaged, all indepen­ and short shafts. Keep this in mind if shipment.) Minor blemishes can often dent movement between them should you are removing a shaft from a heav­ be repaired, but severe damage often be entirely eliminated, and all but the ily corroded coupling, and discuss it warrants replacing the coupling. smallest fore-and-aft movement of with crews before the work begins and Next, the shaft is secured in a lathe, the key would be too much. before they generate an invoice that and the coupling is installed on the Shafts are notorious for seizing in exceeds the cost of a new shaft. shaft, just as it would be in the vessel.

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62 Professional BoatBuilder Left—Fitting and facing involves installing the coupling on its shaft and checking the coupling face’s run-out. For correct alignment, the coupling face must be near perfectly perpendicular to the shaft centerline. If it’s not, the face is milled on a lathe. Right—This coupling has been marked with its after-machining run-out, which does not exceed 0.001" (0.025mm).

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June/July 2019 63 Best Practices: Shaft Couplings

Loose fasteners, movement between coupling faces, or both frequently lead to fretting, which generates iron dust. That, in turn, rusts very quickly, leaving behind a telltale brown halo. Such clues should be investigated promptly.

run-out is found to be too great, the lathe is switched on, and the coupling face is fly-cut, or shaved, to make it truly perpendicular to the shaft. Because coupling bolts are not designed to hold the coupling in the center, it’s essential that during fitting and facing, the coupling pilot bushing should also be checked to make certain it is true, centered relative to the shaft, Then you measure the coupling face or 0.002" (0.025mm or 0.051mm). The and free of damage, and to ensure that irregularity, or run-out, with a dial indi­ larger this irregularity, the greater the it meets the proper dimensional cator, while turning the coupling and imprecision will be when it comes time requirements for engagement with the shaft slowly by hand. Ideally, the run- to carry out an engine/shaft alignment. female recess in the transmission out or lack of perpendicularity between (For more on engine and shaft align­ flange. A heavily rusted pilot bushing, the shaft and coupling face should be as ment procedure, see “The Necessity for instance, may make it impossible to little as possible—no more than 0.001" of Straight,” in PBB No. 159.) If the properly engage the coupling.

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64 Professional BoatBuilder Also beware of undersized pilot over every fastener and set screw, pin, Resources bushings. Ideally you want the female etc. to make sure none is loose. Any pilot to be 0.003" (0.076mm) larger signs of fretting or galling should be than the male pilot. This allows a per­ investigated, along with obvious flaws Buck Algonquin: hydrasearch fect fit during coupling to the engine. such as cracks. Industry professionals recreational.com If it is tighter, there is a chance of bind­ should get into the habit of checking CRC: crcindustries.com/ ing during installation. If it is too couplings for the aforementioned products/heavy-duty- loose, the coupling will not be cen­ flaws, as well as loose or improper fas­ corrosion-inhibitor-10-wt- tered on the transmission flange, teners, or improperly installed seizing oz-06026.html which in turn can lead to run-out and wire whenever they are in the vicinity vibration. of this component, the engine, trans­ Marine Hardware: marinehard More often than not, vibration is mission, or stuffing box. ware.com caused by eccentric or cam motion, Nord-Lock: nord-lock.com which is the result of an off-center About the Author: A former full-service Spurs: spursmarine.com pilot bushing, or coupling bore that is yard manager, Steve works with boat not centered, rather than flange-face builders and owners and others in the Walter Machine Company: misalignment, which induces a con­ industry as Steve D’Antonio Marine waltergear.com/psc.htm stant bow in the shaft. The former is Consulting. An ABYC-certified Master easily identified using a dial indicator, Technician, he sits on that organiza- while the latter will show no abnormal tion’s Hull and Piping and Engine and run-out, again because it is constant. Powertrain Project Technical Commit- High Seas Yacht Service, www.highseas Finally, couplings should be peri­ tees, and is also Professional Boat­ yachtservice.com, in Fort Lauderdale, odically inspected. With the engine off Builder’s technical editor. He acknowl- Florida, for reading this article for tech- (not just in neutral), run your fingers edges assistance from Chris Brown of nical accuracy.

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