Applications on pneumatic:Applications for air motors

Applications for air motors

Air motors are used in a wide range of applications, such as supplying the motive power for winches, cranes, pumps, dispensing machines, stirrers and agitators. The are to be found in drilling rigs (see the chapter on Mining and Quarrying Equipment), on account of their inherent safety.

They are found in a range of rotary hand tools such as drills, screwdrivers, impact wrenches and grinders where their light weight makes them ideally suitable for continuous manual operation.

Hoists and winches

Lifting and hoisting machines can be powered by either air cylinders or air motors. Pneumatic winches are conventional winching mechanisms powered by air motors, although some types of winching action are also performed by air cylinders.

Air motors or cylinders can be used to advantage where other types of driver are less economic to operate or are excluded because of the conditions involved, for example in high temperatures or in explosive atmospheres. An air motor has the advantage of being simple to control. Motor hoists are equally suitable for fixed installations or for running on an elevated rail.

Cylinder hoists

Cylinder hoists have a more limited field of application than motor hoists when working at extreme or variable heights. Cylinders tend to be more bulky than motors of the same capacity, and need to be able to accommodate the hoist length in the overall length of the cylinder unless double or triple extension cylinders or pulley mechanisms are used.

Cylinder hoists are best suited to fixed installations, although they may be trolley mounted. For particular lifting operations, they have an advantage over air motors in that they can provide greater load stability and a more rigid system.

Air motor hoists

The advantages provided by motor-powered air hoists include:

• small, light weight compact construction

• low maintenance costs

• variable lift speeds

• safe if stalled or overloaded

• continuous operation if required

• suitable for use in explosive atmospheres.

The characteristics of a motor hoist or winch are determined by the type of motor fitted. A piston motor would be used when a high starting torque is required; heavy duty or general purpose hoists are usually fitted with piston motors. Vane motors are used where an economical, compact, light weight unit which does not have frequent stop/start cycles is appropriate.

The basic hoist unit can be equipped in different ways for lifting - wire roller or link chain. Control can be by rope or pendant. Rail-mounted units can also incorporate traction drive to provide traverse motion along a horizontal rail and up gradients. An extension of this is the mounting of the hoist on a wire rope suspended from a safety booth above the working area; the hoist runs up and down the wire rope which can also be swung to one side to provide pick-up over a large area. This form of suspension is limited to smaller capacity hoists with a lifting capacity up to 100 kg. The all-up weight of an air hoist can be as low as 1.5% of its lifting capacity.

Air winches

The air winch is an efficient device where any force has to be applied by a rope or cable, such as heavy hauling in mines, workshops, shipyards, ships and construction sites. They are available for applying a force up to several tonnes in useful speeds up to about 20m/ min. Winches have either vane or piston motors with a large number of vanes or pistons to give good slow speed performance, particularly a high break-out torque. Normally a winch will incorporate a reduction gear box built into the drum and an automatic spring loaded brake which ensures fail-safe holding when the air control lever is put into the neutral or if there is no air supply. As with all motors, it is essential that the air supply is lubricated and filtered. Remote control is usually available as an option.

Compressed air starters

Heavy engines, such as ship's diesels, can be started by direct injection of air into the engine cylinders. The air pressure is usually high (up to 40 bar), so special safety precautions have to be taken, which makes this type of starting system difficult to apply to the smaller type of mobile engine. The compressed air for this application is held in a high pressure reservoir and passes via a distributor valve, which is usually mounted on the engine camshaft, to the correct cylinder. A non-return valve in the cylinder head ensures that there is no return flow after the cylinder has fired. The engine can be brought up to speed very quickly and the system is economical in the use of air.

Apart from this specialist application, when compressed air starting is referred to it means the use of an air motor as an alternative to an electric motor, driving a gear on the flywheel rim.

Air motor starters

Any type of air motor can be used as a starter motor- vane, piston, gear or turbine. Most starter motors have been of the vane type, because they are compact units which can be fitted in the space of an electric starter. Indeed some manufacturers offer a pneumatic starter unit as a bolt-on replacement for an electric motor. In small engines, the meshing of the starter pinion is accomplished in the same way as in an electric starter, with a Bendix drive, ie at the commencement of the turning of the motor, the pinion is pushed by a coarse thread into engagement with the rim gear. Figure I shows a typical arrangement.

One disadvantage of the vane motor is that its starting torque is Jess than its slow-speed running torque, but starting a diesel engine requires maximum torque at its breakaway point. Both gear and piston motors have better starting torque characteristics than a vane motor, and theoretically are more suited to this application.

Air starters have an advantage over electric starters in that the motor can be rotated slowly by admitting air through a small pilot bleed until the gears can mesh. Instead of a Bendix drive, the meshing can be accomplished by an air piston and the disengagement by a spring. The operation can be manually controlled, in which case before the full air supply is admitted to the motor, a visual (or audible) check is needed to confirm engagement of the starter gear. Alternatively for standby sets, where it is required that an engine should start without manual intervention, it can be automatically controlled by means of a sensing valve which ensures that the main air valve does not open until there is complete meshing engagement of the pinion.

Air supply for a starter is typically taken from a reservoir charged by the air brake system on a vehicle or by a separate compressor for shipboard installations. For emergency use, the reservoir can be refilled from a high pressure gas bottle.

Air starters compared with electric starters

There are instances when it is an advantage to dispense with electricity on a diesel engine set. There are savings in dispensing with a generator and battery, which are likely to be more than the cost of the air starter and its control equipment. Particularly when the main purpose of running the engine is to drive a compressor, or when compressed air is required for some other purpose, the savings may be considerable.

Air motors are reliable and consistent and generally superior to electric motors in their power/weight ratio. Battery power is much reduced in cold conditions whereas pneumatic motors are unaffected by temperature. If a battery is discovered to be flat, there is little that can be done other than replace it, but an air reservoir can be quickly charged from a source of compressed air.