A Centrifugal Clutch, as it's name implies, gets it's drive pressure from centrifugal force, as opposed to a disc, cone or overcentre clutch that gets it's drive pressure from spring or wedge action.
OPERATION OF A "LEHANE" CENTRIFUGAL CLUTCH
When the clutch centre and shoes are rotated by the driving motor centrifugal force is applied to the shoes and lining. As this force is increased by R.P.M. increase, up to the point where the force over comes the restraining spring pressure, the shoes then throw out on the drive pins and contact the clutch drum that begins to rotate with the driving clutch elements. This stage is referred to as "ENGAGEMENT R.P.M." (Note there is very little power transmission at this stage). As the R.P.M. Increase the power transmission capability increase at the rate of the square of R.P.M. Increase EG.1000 RPM = 2KW, 2000 RPM = 4KW, 4000 RPM = 16KW.
With the above facts in mind a "Lehane" Centrifugal Clutch operates most efficiently when,
- The Idle RPM of drive motor is approximately 75% of engagement RPM
(this prevents shoe Drag and heat generation)
- The Operating RPM of clutch is approximately 25% power requirement of driven unit
(This can be obtained by suitable pulley ratio)
MOTOR TYPE: - eg. DIESEL, PETROL OR ELECTRIC
Motor type is important because a diesel or electric motor usually runs at a more constant RPM under varying loads than an equal size petrol motor. Therefore it is advised to select a slightly larger clutch to allow for the drop in RPM of the petrol motor.
BRAND & MODEL OF MOTOR: - eg. KUBOTA RK70
Through experience it has been found that particular motors have unique characteristics that influence the selection of a suitable clutch. EG, A clutch on a motor with a small flywheel is often more reliable running at higher RPM to reduce rough running and reduce pulsing loads on the clutch. Therefore a smaller pulley selection is advisable.
IDLE RPM OF MOTOR: - eg. 800 RPM
A motor that will idle at low RPM is desirable because the clutch can properly disengage, this eliminates heat buildup by a partially engaged clutch. A Clutch with a low engaging RPM will have more capacity, as can be seen from the selection charts.
Maximum Kw's of motor at max RPM: EG. 15Kw at 3600 RPM & Kw's Required at opening RPM EG. 12.5Kw at 2000 RPM
MAX. Kw's and Kw's at under load operating RPM are important factors when selecting a clutch. Usually the engine does not run at max RPM under operating conditions. More often running at approximately 70% of max RPM in the high torque range. Therefore a motor rated at 5 Kw at 3600 RPM will produce approximately 3.5 Kw at 2500 RPM. A clutch should be selected to transmit 3.5 Kw at 2500 RPM.
TYPE OF DRIVE REQUIRED ON CLUTCH: - eg. Inline,Pulley etc
Often the physical layout governs the type of clutch required. A pulley clutch usually gives flexibility in alignment, centre distance and the ability (By different pulley sizes) to run the engine at optimum RPM. A two piece inline configuration is often a popular choice where cost and simplicity are factors, but accurate alignment and good rigidity are essential. Better option is a ballbearing clutch with a flexible coupling to the driven unit.
SIZE OF PULLEY: eg. 100mm P.C.D.
Pulley diameter require special consideration, not only to obtain correct drive ratio, but physically fit over bearings with a strong enough wall thickness between minor diameter of pulley and outside diameter of bearings (Bearing size is influenced by shaft size etc) Refer to LEHANE selection charts.
NUMBER OF PULLEY GROOVES & BELT SELECTION: - eg. 2A OR 3SPA ETC.
These factors are usually controlled by belt power transmission capabilities and pulley configuration on driven pulley.
SHAFT SIZE & KEYWAY SIZE: eg. 28mm Dia x 7mm Keyway
These dimensions usually controlled by P.T.O shaft on chosen motor. "NOTE" Key should be fitted so as not deform clutch centre, also a small amount of grease will make clutch removal much easier.
TYPE OF SHAFT FIXING: - eg. Washer & Bolt or Grub Screw
Grubscrew is most common with it's simplicity and versatility.
Washer and bolt is used where motor shaft is threaded to take a mounting bolt. This type of fixing is very reliable, but requires a suitable shoulder on crankshaft to clamp against.
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