Grado de cumplimiento

Wear occurs at any time that there is motion between two parts. This motion can be intentional, such as when a shaft rotates in a plain (journal) bearing or when a roller moves back and forth over a track. Wear can also be accidental, where two parts, that should be immovable, chafe together.

If the parts are intended to move together, then the maintenance documentation will have a Schedule of Fits and Clearances, based on the limit system, issued for each mechanism, used on the aircraft.

If the parts are not intended to move together, it will depend upon inspection procedures to discover the problem and repair schemes will be initiated, in an attempt to prevent recurrence.

The Schedule of Fits and Clearances contains tables, which specify the limits on wear and other characteristics such as:

• Ovality (of a hole or shaft)

• Bow of a shaft

The four dimensions, typically covered in wear tables are:

• Dimension New

• Permissible Worn Dimension

• Clearance New

• Permissible Worn Clearance.

Dimension New relates to the size of the part when new, and will show the relevant tolerances.

Permissible Worn Dimension refers to the size to which a part may wear before it must be rejected as unserviceable. Parts, which are not worn beyond this size, can be used again, providing a suitable mating part is chosen to keep the clearance within the permissible figure. This will frequently involve choosing a new part to mate with the worn part.

Clearance New is the desired clearance in limit form. Interference fits are quoted as negative clearances.

Permissible Worn Clearance refers to the maximum allowable clearance when reassembling the component.

6.4.2 Limits for Ovality

This usually occurs as a result of the surface wearing, through friction or linear movement. Ovality and can apply equally to holes and shafts (refer to Fig. 3). Holes may be tested for ovality, using such instruments as Go/No-Go gauges, internal micrometers, or callipers, as were previously discussed in the Tools topic of this course.

A shaft may be tested for ovality, by the use of snap gauges, external callipers and micrometers, which were, again, discussed in the Tools topic.

It is important to test for ovality of a shaft, before testing it for bow, as the results may be suspect if bow is done first.

Bow in a shaft can be determined, in a workshop, by utilising V blocks, a surface gauge and a DTI (in conjunction with a surface table).

6.4.3 Limits for Bow

When dealing with shafts and tubes, it is vital that not only are the ends square with each other, but that the centreline of the complete shaft or tube is straight. If the centre line of the shaft is not straight, then the item is bowed. When the shaft or tube is rotating, especially at a high speed in a bowed state, there is the risk of vibration, which can lead to mechanical failures, loosening of fasteners and (most critical of all) fatigue.

All cylindrical items, both tubular and solid, can be given a limit to the amount of bow permitted. For example a drive shaft, which rotates about 1500 rpm, may have a limit of 0.25 mm (0.01 in) bow over the length of the shaft.

This ensures that, within the limits of production, the drive shafts are effectively straight, giving the least possible vibration.

Hole Wear

Ovality of a Hole or a Shaft Fig. 3

Shaft Wear

Twist is the result of applied torsion on circular or square-sectioned shafts. If the twist disappears, as a result of removing the force, then the shaft will have been loaded below its elastic limit. If the shaft remains twisted, after removal of the load, then it has been loaded above its elastic limit.

The action of a shaft (of whatever section), carrying a torque load is to twist in proportion to the torque applied. The result of cyclic loading of shafts is that, at certain times, the shafts have to be checked for permanent twist. If the shaft has a square section, it can be checked for twist on a surface table using a DTI mounted on a surface gauge.

Solid or tubular shafts that have to be checked for twist will possibly have witness marks or lines engraved or etched at each end of the shaft. The shafts can be checked, by mounting the shaft in V blocks and, then, locating these marks in the horizontal position.

It is possible to measure the amount of twist, to which a shaft is subjected, whilst in operation or rotation, by the use of strain gauges. These emit varying amounts of electric current when under strain, giving an indication (on a calibrated instrument) of the load being applied.

The designer of the aircraft or equipment will set all limits, with regards to the distortion of parts and set them down in the relevant manuals. The methods used to measure the distortion will either be standard procedures, such as using a DTI and surface table etc., or will have a special procedure included in the manuals.

Rivets are a non-detachable form of fastening device, used extensively on aircraft, to secure the items of components built up from sheet metal. They are ideal for forming liquid-tight joints, are cheaper, lighter in weight and are more rapidly fastened than bolts.

Rivets, however, have the disadvantage that they are not really suitable for tensile loads. A riveted assembly cannot be readily dismantled. Rivets, basically fall into two classes, which are:

• Solid rivets

• Hollow or tubular rivets

Rivets are supplied with one head already formed, the tail being formed by hand- operated or machine tools.