Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

PART FOUR: RUDDERS AND STEERING SYSTEMS

Rudder-Bearing Clearance

Because the rudder stock must be free to

turn in the bearings, the bore (inside diam-

eter, or ID) of the bearing must be slightly

larger than the outside diameter (OD) of the

shaft. The stock is almost always of stan-

dard, off-the-shelf diameter rod or pipe. You

can’t normally specify its diameter toler-

ances beyond what’s commercially sup-

plied. The bearing ID is thus specified to

give proper clearance.

The difference between the shaft OD and

the bearing ID is called allowance. The amount

that the machinist can deviate from the speci-

fied ID with the allowance is the tolerance.

For low-speed (low RPM) heavy equipment

(such as rudder bearings), use the following

formula.

Formula 11-8. Rudder-Bearing Clearance

Allowance = 0. 0025 ×(dia., in.)^0.^67

Tolerance = 0. 0025 ×(dia., in.)^0.^34

or

Allowance = 0. 0073 ×(dia., mm)^0.^67

Tolerance = 0. 0211 ×(dia., mm)^0.^34

Where

dia. =shaft diameter

Example: Thus for our 3.75-inch shaft,

we would call for a bearing ID of

Allowance = 0. 0025 ×(3. 75 in.)^0.^67 = 0. 006 in.

Tolerance = 0. 0025 ×(3. 75 in.)^0.^34 = 0. 003 in.

or

Allowance = 0. 0073 ×(100 mm)^0.^67

= 0 .159 mm

Tolerance = 0. 0211 ×(100 mm)^0.^34

= 0 .101 mm

So

Bearing ID = 3. 756 in., + 0. 003 in., –0. 000 in.

(meaning the ID can’t be smaller than 3. 756

in. nor larger than 3. 759 in.)

or

Bearing ID = 100 .159 mm, + 0 .101 mm, –0. 00

mm (meaning the ID can’t be smaller than

100 .159 mm nor larger than 100.260 mm)

UHMWPE-Bearing

Wall Thickness

Minimum bearing wall thickness (for

UHMWPE) should be 0.15 times nominal

shaft diameter. Somewhat greater thickness

is fine, to fit properly inside the bearing hous-

ing wall as needed.

Example: For our 3. 75 inch (100 mm)

shaft, the minimum wall would thus be 0. 5625 in.

(15 mm). Or the minimum bearing OD would be

2 × 0. 5625 in. + 3. 75 in. = 4. 875 in. OD

or

2 ×15 mm +100 mm =130 mm OD

The inside wall of the lower portion of the

rudder port (which houses the bearing)

needs to be machined to be 4. 875 in. (130 mm)

ID, or somewhat larger inside diameter, as

convenient.

The UHMWPE bearing is slid down into

the rudder port, so it needs to be a close slid-

ing fit. Use the following formula to determine

the rudder-port allowance and diameter.

Formula 11-9. UHMWPE Rudder-Port

Allowance Thickness

Allowance = 0. 0014 ×(dia., in.)^0.^67

Tolerance = 0. 0013 ×(dia., in.)^0.^34

or

Allowance = 0. 0041 ×(dia., mm)^0.^67

Tolerance = 0. 0101 ×(dia., mm)^0.^34

Where

dia. =bearing outside diameter

Example:Assume we select a 5-inch

(130 mm) bearing OD. To size the rudder-port

tube, proceed as follows:

Allowance = 0. 0014 ×(5. 00 in. dia.)^0.^67

= 0. 004 in.

Tolerance = 0. 0013 ×(5. 00 in. dia.)^0.^34

= 0. 002 in.

or

Allowance = 0. 0041 ×(130 mm dia.)^0.^67

= 0 .107 mm

Tolerance = 0. 0101 ×(130 mm dia.)^0.^34

= 0 .053 mm

Formula 11-8.

Formula 11-9.