LM-701 SkyHawg

Propeller Speed Reduction Unit (PSRU) Belt Adjustment Procedure.

A large percentage of home built reductions drives are belt driven.  The belts are generally High Torque Synchronous belts, commonly referred to as cog belts or HT belts.  They vary in length and width.   The pitch is generally 8 mm.  The span, distance between center to center of the pulleys, is generally in the range of 10 to 15 inches.  The widths I have seen have been from 25 mm to 100 mm. 

I personally prefer a belt PSRU over other types because of many reasons, the primaries being it's partial relief of  torsional vibration and in the event of a prop strike, the prop, belt, and PSRU take the brunt of the strike instead of the engine. 

Other reasons are;

• Positive, no-slip

• No Lubrication required

• Non Stretch

• Corrosion resistant

• Abrasion resistant

• Virtually no backlash

• Clean operation

• Long life

• Low maintenance

However, all that said, the main reason I am posting this page is to offer an authoritative answer as to the best way to adjust the belts.  Over the years I've heard some say to have the belt somewhat loose while other have said to have it tight as a piano string.   It's a wonder some of us are confused as to the proper procedure.

Recently I was thumbing through a Engineering/Technical Manual  from Rockwell Automation and found the following procedure for adjusting belts.  I believe this is the authoritative answer I have been looking for.  So here it is, draw your own conclusions.  Larry

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