Chapter 2: Installation
49
2.8 Mechanical Drive Systems
Rotary Drive Unit
The rotary drive is coupled to the steering by a chain drive. Most steering
gear manufacturer's supply special autopilot drive attachments (many
include this as standard). The Edson Company is a good source.
Mounting
Having selected the position for attachment of the autopilot drive chain, it
is necessary to determine the chain reduction ratio.
Count the number of turns of the steering gear shaft (driven sprocket)
when the rudder is driven from hardover to hardover. Determine the
sprocket sizes required from the following table.
17
19
25
15
13
17
15
13
17
15
13
15
17
Driven Sprocket
Driver Sprocket
76
76
57
57
57
38
38
38
25
25
25
25
25
1
23456
1
2
3
4
5
6
7
Number of Driven Sprocket
Turns Hardover Hardover
Chain Reduction Ratio
Rotary Drive Chain Reduction Ratios
Type 1
Type 2
D757-1
These ratios provide good steering performance for most vessels. If the
vessel is thought to have unusual steering characteristics, contact our
Product Support Department, or an authorised representative, for advice.
A standard 3/8in. or 1/2in. pitch chain is recommended for the chain drive
and, ideally, the drive sprocket should not have less than 15 teeth. Bore
and keyway dimensions for the drive unit sprocket are detailed in the
following illustration. It is essential that these bore and keyway dimensions
are strictly adhered to. All sprockets must be keyed, set screwed to their
shaft and finally secured with loctite.
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