7.12: Transmission

Question 1

What is gear failure primarily caused by?

Answer 1

Excessive backlash and/or poor lubrication

Question 2

What is overload?

Answer 2

Occurs when the metal has been removed from the gear surface by sliding action, which causes microwell to form from the tiny metal particles

Question 3

What is pitting and spalling?

Answer 3

Caused by thin oil film and high temperatures, causing fine crack and eventually chunks of metal flake away

Question 4

What is crackage or breakage?

Answer 4

Caused by a failure in the heat treatment or improper machining during manufacture. They are often very fine and not visible until the gear has been in service for a while

Question 5

What is interference wear?

Answer 5

Caused by the misalignment of gears or mating teeth, causing wear. Often caused at teeth tips and roots

Question 6

When are wear patterns examined?

Answer 6

Before final assembly
During service
During overhaul
After a failure

Question 7

What is Prussian blue used for in gear teeth?

Answer 7

A dye that is smeared lightly on the leading side of the driving gear teeth, which leaves a contact pattern allowing the correct shimming or assembly relationship to be set up

Question 8

What is backlash?

Answer 8

Used in gears to prevent jamming and making contact on both sides of the teeth simultaneously. It is the amount by width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles

Question 9

The amount of backlash in a pair of mating gears is affected by what?

Answer 9

Change in the centre distance
Variance in tooth thickness
Temperature ranges, which causes differential expansion of the gears

Question 10

What is minimum backlash?

Answer 10

Occurs when all the tolerances give the shortest centre distance and the thickest teeth at the highest point of pitch line runout

Question 11

What is maximum backlash?

Answer 11

Occurs when all tolerances give the greatest centre distance and the thinnest teeth at the lowest pint of pitch line runout

Question 12

What is backlash tolerance?

Answer 12

The allowable amount of backlash

Question 13

What is backlash variation?

Answer 13

The difference between the maximum and minimum backlash occurring in a whole revolution of the larger of a mating pair of gears

Question 14

How is backlash measured in a gear train?

Answer 14

By locking one end of the gear train and measuring the limits of movement at the other end of the gear train using a DTI or feeler gauge

Question 15

What are primary inspection for chains?

Answer 15

Ensures the chain passes smoothly over the sprockets

Question 16

What are secondary inspections for chains?

Answer 16

Requires a load being applied to determine if the chain is stretched or worn. A typical load limit is from 1.5 to 3%

Question 17

What are common materials used for sprockets?

Answer 17

Steel
Alloy
Hard fibre material
Plastic

Question 17

What are sprockets inspected for?

Answer 17

Worn teeth
Misalignment

Question 18

What are drive belts?

Answer 18

They carry much smaller loads to chain drives, quieter, shock resistant and smoother, not requiring lubrication

Question 19

What are screw transmission used for?

Answer 19

Trailing edge flaps
Stabilisers
Rudder trim mechanisms

Question 20

What are three basic screw jacks used in aviation?

Answer 20

The worm and peg mechanism
The worm and nut mechanism
The recirculating ball mechanism

Question 21

How often is the worm and peg mechanism?

Answer 21

Rarely used on aircraft due to excessive backlash which causes localised wear on the peg and worm screw

Question 22

What is the worm and nut mechanism?

Answer 22

Commonly used on aircraft, by using actuation being generated by turning the shaft to drive the nut, or by turning the nut to drive the shaft

Question 23

What is the recirculating ball mechanism?

Answer 23

Uses ball bearings that engage a thread, providing a low friction actuation

Question 24

What are push-pull roads used for?

Answer 24

Aileron and trim tab mechanisms

Question 25

What are the structure of push-pull rods?

Answer 25

Adjustable and lockable threaded end fittings that can have a left and/or right hand thread end allowing the rod to be shortened or lengthened creating a rigid linkage, eliminating the problem of tensioning

Question 26

What is put on a control rod to allow for inspection of whether the adjustable end is sufficiently screwed onto the threaded portion?

Answer 26

A witness hole on the adjustable eye end fitting

Question 26

What are bell cranks?

Answer 26

Used to transmit force or motion, or permit a change in direction of that force or motion

Question 27

What is worn contour cause and correction?

Answer 27

-Cause: Abnormal wear rubbing on guides
-Correction: Reduce load or chain drive speed and provide proper chain lubricant

Question 28

What is pin galling cause and correction?

Answer 28

-Cause: Inadequate lubrication
-Correction: Reduce load or chain drive speed and provide chain lubrication

Question 29

What is tight joint cause and correction?

Answer 29

-Cause: Dirt or foreign substances packed in joints, corrosion or bent pins
-Correction: Clean and re-lubricate the chain, replace the chain

Question 30

What is hooked sprocket teeth cause and correction?

Answer 30

-Cause: Excessive chain wear
-Correction: Replace the chain and the sprocket

Question 31

What is abnormal protrusion or turned pins cause and correction?

Answer 31

-Cause: Excessive internal friction caused by high loading and inadequate lubrication
-Correction: Replace the chain, improve lubrication and eliminate overload conditions

Question 32

What are cracked plates cause and correction?

Answer 32

-Cause: Loading beyond the chains dynamic capacity
-Correction: Replace the chain with a chain of larger dynamic capacity or eliminate high load condition or dynamic overloading