Electromechanical elevators are used for freight, bombs, and stores. In this type of elevator, the platform is raised and lowered by one or more wire ropes that pass over pulleys and wind or unwind on hoisting drums. Hoisting drums are driven through a reduction gear unit by an electric motor. An electric brake stops and holds the platform. The motor has two speeds, full speed and low, or one-sixth, speed. Control arrangements allow the elevator to start and run on high speed. Low speed is used for automatic deceleration as the elevator approaches the selected level. The platform travels on two or four guides. Hand-operated or power-operated lock bars, equipped with electrical interlocks, hold the platform in position.

LUBRICATING SYSTEMS

Most equipment is provided with a lubricating system that supplies oil under pressure to the bearings. The system consists of a sump or reservoir for storing the oil, an oil pump, a strainer, a cooler, temperature and pressure gauges, and the necessary piping to carry the oil to the bearings and back to the sump. The location and arrangement of these parts vary with each piece of equipment. This system allows the lube-oil system to perform the following functions:

0 Supply lubrication to the bearings

Cool the bearings

Flush any wear products from the bearings

The lube-oil pump is generally a gear-type pump. A definite pressure is maintained in the oil feed lines. A pressure relief valve allows excess oil to recirculate to the suction side of the pump.

Quite often, dual strainers are connected in the line so that the system can operate on one strainer while the other one is being cleaned. The tube-in-shell type of cooler is generally used with seawater circulating through the tubes and the oil flowing around them. The temperature of the oil is controlled by adjusting the valve that regulates the amount of seawater flowing through the tubes.

Oil must be supplied to the bearings at the prescribed pressure and within certain temperature limits. A pressure gauge installed in the feed line and a thermometer installed in the return line indicate oil system functioning. Thermometers are often installed in the bearings to serve as a warning against overheating. If there is a decided drop in oil pressure, shut down the equipment immediately. You should investigate even a moderate rise in the oil temperature. An oil-level float gauge indicates the amount of oil in the sump. Some bearings do not require a lot of cooling or flushing of wear products, so they are grease lubricated (like automobile steering joints). These bearings are usually fitted with a zerk fitting (grease fitting), but some may have grease cups installed.

FUNCTIONS OF LUBRICATION

Lubrication reduces friction between moving parts by substituting fluid friction for sliding friction. Most lubricants are oils or greases; but other units, such as water, can be used for lubrication. When a rotating journal is set in motion, a wedge of oil is formed. This wedge (layer of oil) supports the rotor and substitutes fluid friction for sliding friction. The views shown in figure 10-41 represent a rotor (journal) rotating in a solid sleeve-type bearing. The clearances are exaggerated in the drawing so you can see the formation of the oil film. The shaded portion represents the clearance filled with oil. While the journal is stopped, the oil is squeezed from between the rotor and the bearing, As the rotor starts to turn, oil adhering to the rotor surfaces is carried into the area between the rotor and the bearing. This oil increases the thickness of the oil film, tending to raise and support the rotor. Thus, sliding friction has been replaced by fluid friction.

LUBRICATING OILS AND GREASES

Many different kinds of lubricating materials are in use, each of them filling the requirements of a particular set of conditions. Animal and vegetable oils and even water have good lubricating qualities, but they cannot withstand high temperatures. Mineral oils, similar to the oils used in an automobile engine, are the best type of

Figure 10-41.-Rotating journals in sleeve-type bearings.

lubricant for modern machinery operating at high speeds and high temperatures.

Mineral lubricating oils are derived from crude oil in the same process that produces gasoline, kerosene, and fuel oil. They vary according to the type of crude oil and the refining methods used. The same type of oil is usually made in several grades or weights. These grades correspond to the different weights of oil for an automobile, varying from light to heavy.

Oils used in the Navy are divided into nine classes, or series, depending on their use. Each type of oil has a symbol number that indicates its class and viscosity. For example, symbol 2190 oil is a number 2 class of oil with a viscosity of 190 SSU. The viscosity number represents the time in seconds that is required for 60 cubic centimeters (cc) of oil, at a temperature of 130F, to flow through a standard size opening in a Saybolt viscosimeter (fig 10-42).

A 2190TEP oil is used for all propulsion turbines and reduction gears. The letters TEP indicate that the oil contains additive materials that increase its ability to displace water from steel and inhibit oxidation.

Internal combustion engines (gasoline and diesel) use symbol 9110, 9170, 9250, or 9500 lubricating oils.

Figure 10-42.-Viscosimeter tube.

These oils have been developed for lubrication of high-speed, high-output diesel engines.

Grease lubrication is used in locations where the retention of lube oil would be difficult. Some of these locations include throttle links, pump bearings, small boat steering links, laundry equipment, etc. Grease is graded according to its intended use and the additives it may contain. Always be sure that you are using the specified lubricant for the individual machinery part, unit, or system you are responsible for operating or maintaining.

The manufacturer's technical manual for each unit of machinery is the basic reference for the correct lube oil, if no lubrication chart (based on manufacturer's instructions) is available. In addition, the table of recommended oils can be found in NSTM, chapter 262.