Page 301 - Tribology in Machine Design

286 Tribology in machine design

transmitting power in the range of 100-500 kW at a pitch line velocity not
!
exceeding 15ms~ . The usual procedure is to determine both a thermal
rating and a mechanical rating for the unit. The thermal rating tells us how
much power can be transmitted by the unit before its steady-state
temperature is too high. Steady-state temperature over about 120 C can be
considered as too high and sufficient to induce detrimental and permanent
changes in the lubricant. Gears should be run relatively cool to prevent
wear in the long term.
Special lubrication problems are presented by large gear trains transmit-
ting 1000 kW or more. It is usual to employ a pressurized lubrication
system to lubricate, by means of oil jets, the gear teeth and bearings.
Sufficient lubrication is achieved when the oil flow rate is approximately
5
3
1
6.5 x 10" m s~ per 360 kW transmitted. This is applicable in the case of a
single mesh and spur or for helical gears. Other types of gears or different
configurations may require higher rates of oil flow. The location of the oil
jets depends on the pitch line velocity. For the highest velocities, oil jets
should be used on both sides of the mesh; two-thirds of the oil is supplied on
the outgoing side and one-third on the incoming side.
Long service life, free from wear problems depends crucially on the
lubrication system; its ability to keep gears cool, and to deliver lubricant
free from hard particles (filters should filter out particles down to S^m).

8.7. Design aspect of It is accepted in gear design, that with the increase in the pitch line velocity,
gear lubrication the lubricant used should be less viscous in order to minimize the power
losses. In the case of heavily loaded gears, however, it is recommended that
more viscous lubricants are used. Table 8.2 provides some guidance as to
what is the recommended lubricant viscosity for a given pitch line velocity.
The higher values of viscosity are selected for heavily loaded gears.
The next problem is to determine, more or less precisely, the amount of
oil required in the gearbox, as too much oil would cause an increase in
power losses due to oil churning and, on the other hand, an insufficient
amount of oil would adversely affect heat dissipation. The following
formula can be used to find, with sufficient accuracy, the amount of oil
required (in cubic metres)

where N is the power transmitted in kW, Z p is the number of teeth in the

Table 8.2. Recommended lubricant viscosity

Pitch line velocity 1 2.5 4.0 10 16 25
1
(ms' )
Kinematic viscosity 180-300125200100-16070100 5080 40-65
(cSt)