Fluid Coupling Is a centrifugal pump and a turbine built together into a single housing to avoid losses by eliminating piping or channels that would otherwise Be needed to connect them. There is no solid connection between the Pump and the turbine; the liquid, usually oil, transmits the torque by carrying moment of momentum across from pump to turbine. The coupling has two principal advantages: (a) smoothness of operation, since torsional vibrations are not transmitted through it; and (b) the full torque is not developed until the unit is up to speed, which is desirable for both electric motors and internal-combustion engines with heavy inertial loads. Application of the moment of-momentum equation produces the relation between torque developed and change in moment of momentum for either the pump or the turbine. The torque must be the same for both when the operating conditions are steady, since there are no stationary members to absorb torque and no angular acceleration. When the coupling and a portion of the driven and driving shafts are considered as a free body, the angular acceleration is zero for steady conditions; hence, the summation of torques acting on the free body must be zero, and the torque in the driven shaft is exactly the same as the torque in the driving shaft. In the operation of the coupling, consider that the driven member is first stationary and the driving member is rotating at its design speed. Liquid enters the pump near the shaft and is given moment of momentum as it traverses radially outward and flows into the turbine at its outer edge. The moment of momentum of the fluid is reduced to zero in the stationary turbine and the fluid exerts the torque supplied by the driving shaft. As the driven shaft comes up to speed, centrifugal

action in the turbine creates a resistance to flow that reduces the: amount of liquid pumped. No pumping takes place when both shafts rotate at the same speed and, hence, no torque is transmitted. Since the unit is symmetrical, the driven shaft when turning at greater speed than the driving shaft transmits a torque to the driving shaft that in effect provides n braking action. For normal steady operation there must always be a difference in speed, or slip, if torque is to be transmitted. The efficiency e is work out divided by work in, or in which T is the torque, wp the driving-shaft speed, wt the driven-shaft speed, and s the slip, or (wp - wt)/wp.

The larger the diameter of coupling, the less the slip required to transmit a given torque and, hence, the greater the efficiency of the coupling. Since efficiency can thus be increased by simply increasing diameter, no effort is made to curve the vanes or round their leading edges to decrease turbulence or fluid shock upon entering the vanes. Efficiencies are above 95 per cent. In some applications in which variable torque transmission is desired, the amount of oil in the coupling is varied.

Fluid Torque Converter

Is in many ways similar to a Fluid coupling. It has a fixed vane system, however, that transmits torque to the earth, and it always operates completely filled with liquid. For steady conditions of operation there is no angular acceleration, and the summation of all torques acting on the unit must be zero. Since there is a stationary vane system that requires an outside torque T, to hold it fixed, the torques in the driving and driven shafts are no longer equal. For example, if the stationary vanes are curved so that the liquid acts to rotate them in the sense opposite to that of the driving shaft, there is a torque multiplication with the torque in the driven shaft equal to the sum of torque in driving shaft and torque on the fixed vanes, and with a corresponding decrease in speed of driven shaft. By the proper designing of the fixed vane system there may be either a torque multiplication or a torque division. Since no work is done on the fixed vanes, the work output to the driven shaft must equal the work input to the driving shaft less the losses. Maximum efficiency is less than for a fluid coupling, usually between 80 and 90 per cent. As in the case of a reaction turbine, where fixed guide vanes create moment of momentum that is reduced by moving vanes to create torque on a rotating shaft, the fixed vanes of a torque converter are curved to give the liquid moment of momentum. The pump adds to this moment of momentum, and the turbine, by proper design and a speed much less than the pump, takes the moment of momentum out of the liquid and thus has a large torque exerted on it. When large torque multiplication is desired, the torque converter usually is designed with two or more sets of turbine vanes with fixed vanes or pump vanes between them, By use of a freewheeling arrangement on the fixed guide vanes that permit them to rotate

in one direction when torque is applied in that direction, the torque converter becomes a fluid coupling. When there is a large difference in speed of pump and turbine, torque conversion is required and the reaction (fixed) vanes have a torque exerted on them that holds them stationary. As the pump and turbine speeds become nearly the same, the turbine moves so that the fluid discharged from it causes the reaction vanes to freewheel, or take no part in the process, resulting in a fluid coupling with better efficiency than the torque converter.