Tribology effects

1. GAZIANTEP UNIVERSITY
NATURAL AND APPLIED SCIENCE INSTITUTE
MECHANICAL ENGINEERING
CONSTRUCTION AND MANUFACTURING DIVISION
ME 560 TRIBOLOGY
PROJECT OF GEAR AND ROLLING FRICTION
REVIEWED BY ASST.PROF.DR.ABDULLAH AKPOLAT
SUBMITTED BY: BAHADIR KARBA

2. Gear Tribology
• Tribology, derived from the Greek word tribos meaning rubbing, is the science and technology of
interacting surfaces in relative motion and of related subjects and practices. It includes friction,
wear, and lubrication.
• Friction can be directly correlated to gear contact power losses and temperature rise. Gears
generally have a slow continuous wear process which eventually causes loss of gear accuracy.
• Lubrication is the most common way to reduce friction and wear in terms of building up easily-
sheared boundary layers; it is also a common way to transfer heat from the contact zone.
• Choices made during design and manufacturing affect all these aspects. Over the years, tribology
has focused towards smaller and smaller scales in the investigation of these phenomena. This
progress goes along with the tools of surface science.

4. GEAR DRIVESDescription:
All drive systems require a drive gear.
The drive gear is the main transfer from the power source to the driven gear.
A belt from the drive gear to the driven gear is a "belt driven" system.
Another option is the "chain driven" system. The "chain driven" system uses a chain from the drive
gear to the driven gear.
The "gear drive" system is direct gear-drive. The drive gear is directly meshed with the driven gear.
The most reliable means for changing shaft speed remains a mechanical variable-speed transmission.
Power transmission is the movement of energy from its place of generation to a location where it is
applied to performing useful work.
A gear is a component within a transmission device that transmits rotational force to another.These
drives are used to provide a variable output speed from a constant-speed power source or to provide
torque increase for a variable-speed power source.
Gear drives are used in transmissions, rear ends and transfer cases; at times the drive gear will be
smaller than the driven gear. Different gear ratios enable the transmission to shift to lower or higher
rpm speeds.

5. What Are Gears Used For?
Gears can serve as an efficient means to reverse the direction of motion, change rotational speed, or
to change which axis the rotary motion is occurring on. The sizes of the gears usually depend on the
desired gear ratio and the shaft upon which the gears will be mated.
How Do Gears Work?
1. Reversing Direction of Motion: Any two gears that come into contact with one another will
naturally produce an equal and opposite force in the other gear. For example, as the smaller gear
pictured below moves clockwise, the larger gear will naturally move counter-clockwise.
Any shaft attached to the respective gear will rotate in the direction of the gear it is attached to.
2. Changing Rotational Speed: Rotational speed is adjusted through the use of a "gear ratio."
The gear ratio is the ratio of the radius of the drive or "input" gear (the one that is powering the
interaction between the two gears) to the radius of the "output" gear. It can also commonly defined as
the number of teeth on the input gear to the number of teeth on the output gear.
The gear being turned by the motor is referred to as the “driver” gear while the last gear, often the
output gear, in the system is referred to as the “driven” gear. Any additional gears in the drive train
are “idler” gears.
3. Changing The Axis of Rotation: Perhaps the most common gear for changing rotational axis is the
bevel gear (seen below). The bevel gear is commonly used in vehicle differentials to rotate the
motion provided by the engine 90 degrees in order to drive the wheels along their proper axis.

6. Types Of Gear Drives
Classification of Gears: Classification of gears can be done
according to the relative position of the axes of revolution into three
types.
I. Gears for Parallel shafts: Spur Gears, Helical Gears,
Herringbone Gears, Rack and Pinion
Efficiency (%) 98.0 – 99.5
I. Gears for Intersecting Shafts: Straight Bevel Gears, Spiral
Bevel Gears,Screw Gear,Zerol Bevel Gear
Efficiency (%) 98.0 – 99.0
II. Gears for Skew Shafts (Nonparallel And Nonintersecting):
HypoidGears,WormGears
Screw gear (Efficiency 70.0 – 95.0 %)
Worm gear (Efficiency 30.0 – 90.0 %)

7. Advantages of gear drives
• Positive drives.
• High transmission efficiency.
• High velocity possible, even up to 60:1.
• Velocity ratio will remain constant throughout.
• Used for low, medium and high power transmission.
Disadvantages of gear drives
• Require lubrication.
• At very high speeds, produce noise and
vibrations.
• Manufacturing of gears is costly.
• The large number of gear wheels in gear trains
increased the weight of machine.
• Not suitable for shafts having large centre
distance.
• So as usual,designers has to be logical and
optimal during selection of drive.

9. Spur Gears:
• Straight Spur gears are the
simplest form of gears having
teeth parallel to the gear axis.
The contact of two teeth takes
place over the entire width
along a line parallel to the axes
of rotation. As gear rotate , the
line of contact goes on shifting
parallel to the shaft.

10. Helical Gears:
• In helical gear teeth are part of
helix instead of straight across
the gear parallel to the axis.
The mating gears will have
same helix angle but in
opposite direction for proper
mating. As the gear rotates, the
contact shifts along the line of
contact in in volute helicoid
across the teeth.

11. Herringbone Gears:
• Herringbone gears are also
known as Double Helical
Gears. Herringbone gears are
made of two helical gears with
opposite helix angles, which
can be up to 45 degrees.

12. Rack and Pinion:
• In these gears the spur rack can
be considered to be spur gear
of infinite pitch radius with its
axis of rotation placed at
infinity parallel to that of
pinion. The pinion rotates
while the rack translates.

13. Straight Bevel Gears:
• Straight bevel gears are
provided with straight teeth,
radial to the point of
intersection of the shaft axes
and vary in cross section
through the length inside
generator of the cone. Straight
Bevel Gears can be seen as
modified version of straight
spur gears in which teeth are
made in conical direction
instead of parallel to axis.

14. Spiral Bevel Gears:
• Bevel gears are made with
their teeth are inclined at an
angle to face of the bevel.
Spiral gears are also known as
helical bevels.

15. Zerol Bevel Gears:
• The teeth of "Zerol" bevel gears are
curved but lie in the same direction as the
teeth of straight bevel gears. "Zerol" bevel
gears are similar to spiral bevel gears
except the "Zerol" gears have zero spiral
angle and are manufactured on the same
machines as spiral bevel gears.
• As in straight bevel gearing, "Zerol" bevel
gears have no inward axial thrust. Their
zero degree spiral angle produces no thrust
load. These two types of bevel gearing are
interchangeable in equipment; thus, no
changes are required for thrust bearings.

16. Hypoid Gears:
• The Hypoid Gears are made of
the frusta of hyperboloids of
revolution. Two matching
hypoid gears are made by
revolving the same line of
contact, these gears are not
interchangeable.

17. Worm Gears:
• The Worm Gears are used to
connect skewed shafts, but not
necessarily at right angles.
Teeth on worm gear are cut
continuously like the threads
on a screw. The gear meshing
with the worm gear is known
as worm wheel and
combination is known as worm
and worm wheel.

18. Planetary Gears
• Note that the sun gear, planet pinions, and ring
gear are constantly in mesh. Planetary gearing
or epicyclic gearing provides an efficient
means of obtaining a compact design of power
transmission with driving and driven shafts
parallel to each other. Planetary gear units can
use spur or helical gear tooth forms.
• Planetary gears are suitable for installations
requiring a/an:
• Increase or decrease in speed of the driven
component
• Change in direction of rotation of the output
• Torque increase or decrease
• As shown in Figure 36, planetary gears are
similar to the solar system. The planet pinion
gears or carriers each turn on their own axis
while rotating around the centrally positioned
sun gear. The planet pinion or carrier gears
mesh with the inside gear teeth of the ring gear.
Figure 36: Planetary Gears

19. • The planet pinions are mounted on shafts in the carrier assembly
and can rotate on their axis to walk around the sun gear or the ring
gear.
• When power is applied to drive the sun gear, on either the planet
pinion carrier or the ring gear, the entire planetary system will
rotate as a unit.
• A restraining force (reactionary device) applied to one of the other
two planetary members will hold the system stationary. With no
reactionary device in place, a neutral situation results in the drive
unit.
• When power is applied to one member of the planetary system
and a brake mechanism is applied to restrain a second member
from rotating, the remaining part will become a power output
source as illustrated by the following examples.
• When the sun gear is driven, as shown in Figure 37, and a brake is
applied to the ring gear, the planet pinions walk around the ring
gear, forcing the planet pinion carrier to rotate in the same
direction as the sun gear, but at a slower speed.
Figure 37: Planetary Gear Movement

20. • When the planet pinion carrier is driven,
as shown in Figure 38, and a brake is
applied to the ring gear, the planet
pinions revolve around the ring gear,
forcing the sun gear to rotate in the same
direction at a higher speed.
Figure 38: Planetary Gear Movement

22. Advantages Of Planetary(Epicyclic) Drive
• Compared to conventional gearboxes has
smaller dimensions
• Easier to sort through the constant rounds
of shot
• Greater durability than conventional bikes
in gear
• Easy to achieve high transmission ratio due
to the size
• They have higher gear ratios
Disadvantages Of Planetary drive
• More expensive than conventional production
of gearboxes
• More complex than conventional transmission

23. Rolling Friction
• It is much easier to roll surfaces than to slide them.
• Rolling friction is the resistance to motion that takes place when a surface is rolled over another surface.
The term rolling friction is usually restricted to bodies of near perfect (continuous) shapes with very small
surface roughness.
• With hard materials, the coefficient of rolling friction
between a cylindrical or spherical body against
itself or a flat body generally is in the range of 5 × 10−3 to 10−5
• In comparison, the coefficient of sliding friction of dry bodies
ranges typically from 0.1 to sometimes much greater than 1.
• During rolling of two surfaces relative to each other,
any relative motion can be regarded
as a combination of rolling, sliding and spin (Johnson, 1985).

24. Cause of rolling friction
When an object is rolled on a surface, certain things happen:
1.The object is deformed at the point of contact with the surface.
2.The surface is deformed at the point of contact with the object.
3.Motion is created below the surface as a result of the above mentioned points.
The primary cause of this friction is that the energy of
deformation is greater than the energy of recovery. Also,
there is an adhesive force between the two surfaces which
needs to be overcome constantly. The amount of friction
is based on a variety of factors, such as,
•The quality of the sliding body
•The quality of the surface,
•Load
•Diameter of the rolling object
•Surface area of the body

25. Surface irregularities
The surface of the wheel and what it is rolling on are not perfectly smooth. They have irregularities.
Close-up showing surface roughness
This surface roughness is a reason for the resistance to rolling motion. It causes a "jiggle" when the wheel is rolling.

26. Coefficient of rolling friction
Examples of the coefficient of rolling friction include:
Train wheel on steel track: 0.001
Ordinary car tire on dry pavement: 0.015
Truck tire on dry pavement: 0.006-0.01
Rolling friction equation
• The general equation for rolling friction is:
• Fr = μrN
where:
• Fr is the resistive force of rolling friction
• μr is the coefficient of rolling friction for the two surfaces (Greek letter "mu" sub r)
• N is the normal force pushing the wheel to the surface
• This equation is a simple version of the resistance to rolling motion. More complex versions
include the effects of wheel diameter and speed.

27. Friction in Rolling Element Bearings
Bearing selection based on load and expected life was explained in previous subheadings. Sometime,
friction plays role in bearing selection. A rough estimation of frictional coefficients(f) is :
• Self-aligning ball bearings, f = 0.0010
• Cylindrical roller bearing, f = 0.0011
• Thrust ball bearings, f = 0.0013
• Single-row deep-groove ball bearings, f = 0.0015
• Tapered and spherical roller bearings, f = 0.0018
• Needle bearings, f = 0.0045

28. o Coefficient of friction is represented by symbol f.
o The value of f is a system property and will change with kind of lubricant, applied load, and
mounting.
o The values quoted are idle values.
o The lowest coefficient of friction is 0.001 and highest coefficient of friction is 0.0045.
o From this comparison it can be concluded that self aligning bearings are preferable choice.
Why coefficient of friction in needle roller bearings is higher than other bearings ?
• Considerably greater length than dia.
• Rollers cannot be manufactured as accurately as other cylindrical rollers.
• Rollers cannot be guided well.
• Rubbing action against each other.

29. • Anti-friction bearings is a common terminology used by a number of authors for rolling element
bearings. This means bearings which go against friction and almost negligible friction. But we are
trying dig out all possible sources of friction in these anti-friction bearings.
• One common cause of friction in all rolling element bearing is hysteresis losses. Such losses occur
due to loading and unloading of rolling element on rings.
• The lowest value of coefficient of friction due to hysteresis happen in chrome steel, therefore,
chrome steel is used as one of materials for rolling elements.
• It is necessary to understand ball/roller are subjected from zero load (as shown in green color in
Fig. 6.39(b)) to maximum (as shown in red color in Fig. 6.39(b)) load. Due to such load variation,
elastic deformation of rolling elements change from zero to maximum as shown in Fig. 6.39(a).
Fig. 6.39: Loading and unloading of rolling elements.

30. • Total friction = Frictionload + Frictionspeed + Frictionseal
• The total frictional moment of a bearing is obtained by adding the frictional moment M0, which is
independent of the bearing load, and the load-dependent frictional moment M1.
• For sealed bearings and axially loaded cylindrical roller bearings, additional components of the
frictional moment must be taken into account.
• Sliding friction occurs at guiding surfaces of rolling elements in the cage, at the roller faces and the
raceway lips, and between adjacent rolling elements in cage-less bearing. Internal friction &
churning action of lubricant cause friction.

31. Sources of Rolling Friction :
• It is important to know the source of rolling friction, so that proper actions may be implemented to
control the rolling friction. Let us consider a hard steel ball which rolls over a softer rubber such as
shown in Fig. 2.20.
• As it rolls along, the ball displaces rubber elasto-plastically around and ahead of it. The force
required to display rubber is almost equal to the observed rolling friction.
• Thus, the rolling friction is essentially a measure of the force required to deform other material.
• With a very bouncy rubber rolling friction will be lesser compared to a very soggy rubber.
The main contributions to friction in rolling contacts are :
1. Micro-slip effect within the contact area.
2. Elastic hysteresis of the contacting materials.
3. Plastic deformation of the materials, and
4. Adhesion effects in the contact.
Fig. 2.20: Rolling friction in rubber.