In addition to. the types of hydraulic ele vators mentioned above, there are the so-called aero-hydraulic and hydro-steam elevators. The elevator proper may be of any one of the hydraulic types already described, but they differ in respect to the manner in which the hydraulic pressure is obtained. In the aero hydraulic machine a tank partly filled with water is connected to a supply of air pressure. The elevator ascends by simultaneously admit ting air into the tank and water into the cylinder. For the descent, the air is dis charged, while the machine returns the water to the tank. In the hydro-steam elevator, steam is employed in place of air. All hy draulic elevators absorb the same amount of power in each cycle of their operation, inde pendently of the live load in the cage. The power consumption for this reason, notwith standing an excellent mechanical efficiency, is quite large. In addition, the speed of the hy draulic elevators varies largely with the load in the car.
Electric Elevators.—These excel the hy draulic elevator by reason of a speed practi cally independent of the load and by a lower power consumption. They may .generally be classed as having machines with winding drums or machines with traction sheaves. Further, distinction is made in respect to the type of gearing employed, into worm gear, worm and spur gear, herring-bone gear, gearless or 1:1 and 2:1 gearless machines.
Fig. 6 shows a typical worm gear machine with winding drum. The motor in the illus tration is of the direct current type, but may be alternating of single or polyphase. The brake is placed between motor and gear hous ing and is of the shoe type with springs to hold the shoes in frictional contact with the brake pulley. The brake is released only upon the admittance of current to the brake magnet and is applied immediately upon the interrup tion of the current supply thereto. 'The hoist ing cables lead from the cage to the face of the drum, where they are solidly anchored. At the opposite side of the drum a counterweight is attached, adapted to counterbalance the cage and live load. Frequently also a second coun terweight is employed, suspended directly from the cage. The weight of the counterweight where one is used, or their aggregate weight where two are used, is made to exceed the weight of the cage by an amount of from 30 to 50 per cent of the maximum load. On ac count of this arrangement a relatively small motor can be employed since it will be subject only to from 70 to 50 per cent of the maximum load when lifting the same. When lowering the empty cage the load on the motor corre sponds to from 30 to 50 per cent of the maxi mum load. The worm runs partially sub merged in oil (preferably castor oil) and owing to the excellent lubrication the efficiency of worm gearing is higher than usually antici pated. The roping employed in a traction ele
vator installation is shown diagrammatically in Fig. 7 and will also be evident from Fig. 8. The machine is usually and preferably located overhead. The ropes pass from the car to the traction sheave, thence to an idler or second ary sheave and again over the traction sheave to the counterweight. The tension due to the weights of car and counterweight and the approximately two half wraps of contact be tween the ropes and traction sheave furnish the necessary adhesion to transmit motion from the traction sheave to the elevator without slip. This adhesion is instantly destroyed if either car or counterweight is obstructed in its de scent, in which case motion of the elevator must cease even though the machine keeps on revolving. This property is a most valuable safety feature of the machine with traction sheave. By arranging the car to land on an oil buffer at the lower landing and by simi larly obstructing the further descent of the counterweight when the car is at the upper landing, the car travel is absolutely fixed be tween two limiti. Another advantage of the traction machine lies in the fact that the width of the traction sheave is independent of the height of the building. For a given capacity therefore, a standard machine can be provided, irrespective of the elevator travel. In the 1:1 traction machine (Fig. 7) the traction sheave is mounted directly on the motor shaft. As the illustration shows, the construction of the machine is simplicity itself. It consists of an armature with extended shaft, carrying a brake pulley and a traction sheave, all sup ported on two bearings. At full speed the motor runs at about only 60 revolutions per minute. Reductions in speed, are obtained by means of field control and by manipulation of resistance in series and parallel with the arma ture. Contrary to popular belief motors for such low speeds can be made with as high efficiency as high speed motors, although, of course, the motor frame assumes considerable dimensions. Owing further to the absence of gears, the 1:1 traction machine has the highest efficiency of any elevator machine yet designed. Wherever it has replaced existing hydraulic elevators,. the saving in power consumption has paid for the new installation within a few years. The microdrive machine shown in Fig. 8 de rives its name from the fine adjustments in the stopping, which can be made with this machine in a manner not unlike the action of a microm eter.