Page 260 - The Tribology Handbook

Sealing against dirt and dust B20

When operating in dirty and dusty conditions, the reliability of equipment depends almost entirely on the amount of
abrasive material present. Natural soils contain abrasive materials in amounts varying from 98% down to 20% by weight.

Table 20.1 The source, nature and effect of contaminants
Operating conditions, effect on reliabilip and basic requirements
Source Nature os contaminant
Wet (more than 15% by
weight of water) Dry Wet and dy
~ ~ ~~~
Contact with soills Sharp faceted grains Severe, highly abrasive. Some loss of reliability. Severest, maximum
(high silicon) predominantly silica Considerable loss of Machinery ingesting abrasive effect. Very
($30,) 98% by weight reliability unless air requires efficient good air cleaning and
occurs frequently extensive sealing air cleaning sealing required
provided
(low silicon) Predominantly grains of Clogging rather than Little loss of reliability. Poor conditions. Good
Calcium (CaO). Silica abrasive. Some loss of Good air cleaners and air cleaning and
less than 25% by reliability. Sealing sealing required sealing required
weight. required
Airborne dust Any finely divided - Reduction in reliability -
material in the dry dependent on dust
state picked up in air concentration. Very
currents large air cleaners
required for highest
concentrations

DESIGN OF SEALING SYSTEMS
Design to reduce the effects of dirt and dust

1. Keep to a minimum the number of rotary or sliding mechanisms as reciprocating hydraulic motors and
parts exposed to bad conditions. exposed hydraulic rams for earth moving equipment.
2. Provide local clean environments for bearings and Abrasive material is bound to enter the ram system
reciprocating hydraulic mechanisms by means of sep- which will be highly destructive to precision mechan-
arate housings or sealing arrangements. isms. Provide independent fluid systems.
3. Provide adequate space in the sealing arrangement for 9. For mechanisms relatively crude in function where
oil lubrication. lubricant retention of any sort is either too costly or
4. Do not use grease lubrication for bearings, unless impracticable, load carrying bearings and reciprocat-
design for oil becomes uneconomic. ing parts may be made in material with very hard or
5. Provide adequate means for replenishment of lubri- work hardening contact surfaces. Austenitic man-
cant; easily accessible. ganese steels have work hardening properties, but are
6. Protect lubrication nipples locally to avoid erosion or not readily machinable. The shape of parts must be
fracture from stones and soil. arranged to be used as cast or with ground surfaces.
7. Provide psitive means for checking amount of lubri- 10. Arrange the position of air cleaner intakes to avoid
cant in housing. locally induced dust clouds from the motion of the
8. Never use a common hydraulic fluid system for such mechanism.

B20.1