ISO 9001:2008 Certified
For Training Purpose Only
MODULE 7
Sub Module 7.12
Contents
TRANSMISSIONS --- 1
INSPECTION OF GEARS AND BACKLASH --- 2
INSPECTION OF BELTS AND PULLEYS --- 3
CHAINS AND SPROCKETS --- 4
INSPECTION OF SCREW JACKS --- 6
LEVERS DEVICES --- 8
ISO 9001:2008 Certified
For Training Purpose Only
TRANSMISSIONS
The various types of transmission mechanisms were described in Module 6 (Materials and Hardware). Here, in Module 7, consideration is given to the inspections relative to such transmission mechanisms as:
Gears
Belts and Pulleys Chains and Sprockets Screw Jacks
Levers
Push-Pull Rod Systems.
ISO 9001:2008 Certified
For Training Purpose Only
INSPECTION OF GEARS AND BACKLASH
Gears can be found in a wide variety of applications, throughout aircraft and engine installations. The most common applications are those used to reduce or increase the rotational speed of an input shaft.
Some aircraft engines utilise a reduction gearbox to slow the main shaft speed to a lower figure for either the propeller or the fan. Accessory drive gearboxes, actuators, flap mechanisms and some flying control runs use gears of one form or another. Wear in gears, assuming that the bearings are not worn, will be found on the faces of the gear teeth. This wear shows up as excessive backlash, which can be easily found by simply rotating the input mechanism, such as the main drive shaft, whilst holding the output rigidly. It should be borne in mind that a small amount of backlash is essential for the correct operation of bearings.
By rotating the input in both directions, the angular difference, when measured, is the accumulation of wear of all the gear teeth. The maintenance manual should give the total backlash figure for the particular train of gears, to indicate whether the train is serviceable or if further inspection is needed.
Because the majority of gears, whether in a train or a single pair of gears, are within some other component, any signs of wear or other problems are usually solved by replacement of the major component.
INSPECTION OF BELTS AND PULLEYS
It is rare to find belt drives being used on modern aircraft. Due to the risk of slippage once the belt has taken on a slight stretch, there has to be some method, often automatic, to retain the set tension over a long period.
In some installations, the drive from the high-speed engine to the low-speed propeller is accomplished by the use of a ‘toothed’ belt drive. The teeth on the inside of the belt engage with grooves machined onto the drive (and driven) pulleys. This reduces the chance of slippage. Most piston engines on smaller aircraft have a belt drive to the ac generator or the vacuum pump, similar to that found on many motor cars.
The same maintenance applies to most belts, in that the security is checked before each flight. The belts must also be checked, at regular intervals, for signs of wear, by fraying and splitting, in addition to a tension check on the either the belt itself or the tensioning mechanism. The majority of belts (whatever their usage), have a finite life and are also subject to ‘on-condition’ monitoring.
ISO 9001:2008 Certified
For Training Purpose Only
CHAINS AND SPROCKETS
Chain assemblies should be inspected at the specified frequency, which is laid down in the aircraft maintenance manual. One of the major checks should be that the chain passes smoothly over the sprockets. If there is any roughness or binding, then further checks will be required.
Another major check of the chain assembly is that for wear, which involves applying a load to detect whether the chain is worn. The load can be applied to a free length of chain where the amount of deflection can be measured and compared to a limit published in the manuals. An alternative would involve pulling the chain at a pulley and seeing if the chain can be lifted from the sprocket by a significant amount.
Failure of either of the preceding checks, followed by a check for chain elongation, would require replacement of the chain. A typical percentage limit of elongation could be 2% when the cleaned and dried chain is loaded with the correct tension. Chains should also be checked for normal faults that can befall most mechanisms; these include damage, corrosion, cleanliness and insufficient lubrication.
One other inspection, which could be done on a chain assembly, might be for correct articulation. This check involves the chain being drawn over the plain shank of a screwdriver. Tight joints, found by this method, should be carefully inspected and the chain rejected if there are any doubts as to its serviceability.
ISO 9001:2008 Certified
For Training Purpose Only
INSPECTION OF SCREW JACKS
There is little maintenance to be carried out on screw jacks, apart from regular greasing of all the exposed threaded portions and checks for backlash between the ‘nut’ and the screw. Some screw jacks are in the form of actuators and some are used to drive flaps and other aerodynamic devices.
A screw jack actuator (refer to Fig 1) is a fairly simple design and correct maintenance is vital because it is part of the flying controls. The actuator has a grease nipple fitted, which allows not only the bearings to be kept well lubricated, but the screw will also pick up some grease when the actuator is extended and retracted.
In-service checks, other than lubrication will probably only include backlash checks on the actuating shaft. These will probably mean no more than a side-to-side hand movement of the rudder trim tab, which ensures that the movement is not excessive.
Another form of screw jack is that used to drive flaps up and down. This form of jack will usually be found with a drive gearbox, transmitting the motive power to the screw and ‘ball nut’, that connects to the flap structure.
Screw Jack Actuator
Fig. 1
Threaded Screw Shaft
Thrust Bearings
Control Input
The rotation of the flap motor drives a number of the gearboxes simultaneously, which transmit rotation to their respective screws. The ball nuts are all driven down the screws and these push the flaps to their selected position.
Because the screw jacks and their ball nuts are exposed to the elements, it is essential that all checks and lubrication required must be thoroughly done.
As previously mentioned, the screw jacks must be thoroughly greased but only after all dirt, sand and other materials have first been completely removed.
The nuts will be checked for wear at regular intervals and this check will probably require special tools and measuring jigs. On a day-to-day basis, the backlash on the nut/screw combination can be checked by an up and down movement of the flap trailing edge.
ISO 9001:2008 Certified
For Training Purpose Only
LEVERS DEVICES
Levers can be found in numerous places within an aircraft and maintenance of these items can vary, depending on their location and purpose. As a rule, levers will be used to transmit thrust from one medium to another. For example, a push/pull system may drive a lever that operates a service, with an increase or decrease of mechanical advantage or a change of direction.
Apart from the bearings of the lever requiring lubrication, (unless they are sealed-for-life bearings), there is little maintenance required, other than physical checks for damage, distortion and cracks.
PUSH-PULL ROD SYSTEMS
The majority of aircraft push-pull systems can be found in both the flying and engine controls (refer to Fig. 2). They may consist of a series of hollow aluminium tubes, which have either fixed or adjustable end fittings. Sometimes, to prevent the tubes vibrating, their length is kept short and idler levers are fitted between each pair of tubes. As an alternative, rollers or bushes can be installed along the length of the push-pull tubes to provide support.
The maintenance required for this type of control consists generally of inspection and rigging. As the bearings in both the idler levers if fitted, and the end fittings are normally sealed for life, the only inspections to carry out are for signs of damage and overheating. If the pilot complained of stiff controls, then a check of each bearing assembly would be required, to check which bearing was stiff.
Rigging of push pull rods is relatively simple. The rigging pins hold the rods and levers in the datum position and the adjustable ends are altered until all the connecting bolts can be inserted without any force being required.
