ATR3-b1′s clutch  is depressed and retracted by a double-acting 7″ pneumatic cylinder fed by a pair of Pressure/Exhaust (P/E) valves that deliver a pressure differential 0f plus or minus 85psi between the cylinder ends.  (The pair can also deliver a pressure differential of 0, but this mode is not utilized.)  The cylinder’s 3/4″ bore can therefore deliver pi*(3/4″/2)^2*85psi = 38lb force when either retracting or extending.  This should be enough to disengage ATR3′s clutch, though a larger or second cylinder would be necessary to actuate the full braking force.

ATR3-b1: Clutch actuated

The cylinder is affixed to a 1/8″ aluminum plate bolted to 1/2″ hollow aluminum tubing which is affixed to ATR3′s framework at a 30 degree angle.  The “claw” at the end of the cylinder’s actuated rod allows the clutch pedal to move up and down during the linear travel, which is necessary to accommodate the rotating path of the clutch.  Mounting the air cylinder on a freely rotating gimbal would probably be a more robust solution.

Since the clutch is actually single-acting, returned by a tensioner spring attached to a drive belt pulley beneath the tractor chassis, it would seem that the cylinder should not need to be double-acting.  However, when conducting experiments with the cylinder configured as single-acting, the cylinder was able to maintain a fixed clutch position but had difficulty overcoming the static friction in the tensioner spring at various clutch positions.  Given the age and somewhat rusted condition of the tensioner spring that the clutch releases, the tensioner spring was removed altogether and the cylinder modified for double-acting operation.  This has the added advantage of defaulting to a clutch-disengaged state when power or air pressure is lost.  The cylinder’s ability to maintain sufficient belt tension for motion is verified in the experiment below.