TUFNOL Technical information 

STAVE TYPE BEARINGS

FOR MARINE APPLICATIONS

For many years Bear Brand TUFNOL has been used for water lubricated bearings in marine applications. Bear Brand TUFNOL stern tube and rudder bearings have been installed on a great many vessels of all sizes and approved by the major marine classification societies.

For large bearings it has been conventional practice to make stave type bearings, where the housing is lined with machined strips cut from sheet to form the bearing (see Figure 1).

 Figure 1.  Fitting a stave bearing.

These strips or staves are held in place by being wedged between key strips which are secured to the bore of the housing. When fitting, the last stave to be inserted between a pair of key strips is usually trimmed on width to give a tight wedge fit and is then driven in to hold all of the staves in place. By machining chamfers on each stave, when assembled, grooves are formed which allow transport of the water lubricant into the bearing.

  Figure 2. Types of groove.

Two types of water grooves are illustrated in Figure 2 and these are produced by simply machining the edge of each stave. The “UV” shaped groove suffers less area reduction due to bearing wear, its rounded base encourages a more efficient scouring effect to remove sand particles, and the chamfers adjacent to the bore encourage the formation of a water film between the bearing and shaft. The “UV” shaped groove is recommended for stern tube bearings. The simple “V” shaped groove is usually adequate for rudder bearings.

The entry of sand into stern tube bearings can be prevented to a great extent by the provision of a water supply to the inboard end of the bearing. This practice is required by Lloyd’s Register of Shipping. A pressure of approximately 0.35kgf/cm2 (5lbf/in2) above outside water pressure at maximum draft is considered adequate.

Apart from its function as a lubricant, water also acts as a coolant. This important factor contributes greatly to the excellent performance of TUFNOL bearings at high speeds and heavy loadings.

On the other hand, a water starved stern tube bearing may generate sufficient heat to damage the laminate. However, tests have shown that failure of the water supply is not likely to damage the shaft even when the heat generated is sufficient to char the bearing.

Oils have no detrimental effect on TUFNOL laminates and may be used to lubricate bearings which are not suited to water lubrication. It must be remembered however, that oils do not possess the cooling properties of water and should not be used in bearings which generate running temperatures in excess of 800°C.

In the case of open-ended water lubricated bearings, the use of greases and heavy oils is not recommended. Grease will clog water grooves, thus restricting the waterflow. Grease also tends to trap sand particles, resulting in the formation of an abrasive paste.

DIRECTION OF LAMINATIONS

The expansion of Bear Brand TUFNOL due to water absorption is greatest in the direction normal to the laminations, i.e., perpendicular to the plane of the sheet. At first sight, therefore, it might seem better to arrange the staves edgewise to the shaft as shown in Figure 3.

 Figure 3. Incorrect direction of laminations.

 Figure 4.  Correct direction of laminations.

This would permit the use of extremely small initial clearances.  However, such an arrangement requires a more complex design to prevent the large swelling in the circumferential direction causing dangerously high compressive hoop stresses. Instead the arrangement shown in Figure 4 is recommended, as no precautions need to be taken to limit peripheral stresses except for the usual accuracy in fitting the final stave. However, bearings have been successfully installed with perpendicular laminations and can be supplied if necessary.

STAVE DIMENSIONS

For economic manufacture, staves should ideally be about 75mm to 100mm wide but the exact width needs to be calculated accurately to ensure that the stave will fill the circumferential space between the keys, plus a small allowance for trimming when hand fitting. This is done by calculating the total angle occupied by the keys, subtracting this from 360 degrees and then dividing the remaining angle of arc by a suitable number of staves.

Typical example:

A rudder bearing is required to fit into a housing of outside diameter (D) 550mm with a shaft diameter (d) 500mm. There are 2 metal key strips fixed at 180 degrees and the width of each key strip is 50mm (see Figure 5).

 Figure 5. Calculation of Stave Dimensions.

   Basic formulae for Figure 5:

   W/D = Sin(È/2)

   t = (D – (d * Cos (È/2))) / 2

Using the formulae shown in Figure 5, the angle subtended by each key is 10° 26' so the arc of cir­cumference to be filled by staves is: -

360 - (2 x 10° 26') = 339° 8'

Since the two keys are evenly spaced, an even number of staves will be required. It is found that 20 staves each with a theoretical angle of 16° 57' gives a stave width of 81.09mm which is acceptable. To this must be added a small allowance for hand fitting to allow for inaccuracies in the housing and cumulative errors. Adding 0.8mm to each stave, the machined width of stave becomes 81.89mm, which means the actual angle to which the stave will be machined is 17° 8'.

The bearing clearance after fitting is calculated from the formula:-

clearance = (0.025 x D) - (0.0237 x d)

which in this case gives 1.9mm clearance and swelling allowance.

The thickness of material required to produce the staves is 26.85mm plus.

FITTING STAVE BEARINGS

The most common method of fitting TUFNOL staves is to place all but one of the staves in a section of the bearing between key strips, holding them in position temporarily by means of wedges in the remaining space. The width of the last stave is then adjusted by removing the material to suit the gap.

The fit obtained should be such as to allow the final stave to be driven home without undue force. A heavy steel bar, having a short lip or hook at one or both ends is a useful tool for removing any stave which is too tight a fit, thus requiring further slight adjustment on width.

CLEARANCES

With stave bearings fitted in the recommended manner, the initial clearances should not be less than those shown in Figure 6. In addition to the appropriate clearance in the bearing bore, it is also advisable to allow an axial clearance of 2.5mm per metre of bearing length.

 Figure 6. Recommended clearances for Bear Brand TUFNOL Stave Bearings.

Notes to Figure 6.

Use Curve A for all rudder bearings and stern tube bearings for new buildings and vessels not entering service for a period of approximately 3 months.

Use Curve B for Stern tube bearings only, for vessels returning to service immediately.

Shaft diameter (mm)	Wall thickness (mm)	Shaft diameter (in.)	Wall thickness (in.)
25	3	1	1/8
50	6	2	1/4
75	9	3	3/8
100 - 175	13	4 - 7	1/2
200 - 250	19	8 - 10	3/4
275 - 375	22	11 - 15	7/8
400 and above	25	16 and above	1

Clearances indicated in the graph are based on bearing wall thicknesses shown in the above table. Users of bearings having a substantially greater wall thickness are recommended to consult Tufnol Composites Ltd. As a general guide for non-standard wall thicknesses, clearance can be found from:

Diametral Clearance = (0.025 x Housing Diameter) - (0.0237 x Shaft Diameter)

Click here for Bear Brand TUFNOL page

Click here for main TUFNOL Bearings technical page