The mechanical strength of the varoius plastic materials is obviously
lower than that of metallic materials, but the numerous benefits of
engineering plastics should always be borne in mind:
PA6.GF Akulon GF
Extruded polyamide 6 resin with addition of 20 % glass fibre and graphite. Thanks to the glass fibre the material is very resistant to abrasion, compression and tensile stress. Akulon GF is highly recommended for gear constructioni.
PA6 Akulon 6
Compared to Akulon 6 + glass fibre, it has less compression strength and tensile stress resistance, but it has superior damping power so should be used if this feature takes priority.
PA 66 Akulon 66 - PA 66.6 Akulon GX
Features are very close to those of PA6, but with greater toughness. Akulon GX is to be preferred when higher UV resistance is required.
PA6 G - PA6 G HR - PA6 G OIL Omniamid
Cast polyamides provide the possibility to produce semifinished products, or hollow rods, of large dimensions and are used mainly for big diameter gears. The oil-filled nylon is self-lubricating and abrasion resistant and is particularly suitable for big gears on earth moving equipment or cableway systems.
PET Arnite
Material with the best dimensional stability, and particularly suitable in the application of precision gears immersed in water or other liquids. It has a low coefficient of friction, while maintaining a low shock resistance.
POM - Acetalica
It also has good dimensional stability and good chemical resistance, the material is self-lubricating, but the shock resistance is lower than that of the polyamides. It is usually used for precision and small gears.
PP - PE 500 - PE 1000
The polyolefines are materials with low mechanical resistance, to tensile, compression so they are not recommended for gears, but they are used when good chemical resistance and low transmission powers are required. PE 500 is tougher than PE 1000 but less resistant to shocks, so when choosing from among the different grades of PE you have to bear these variables in mind. Among the polyolefins PP is the material with the highest tensile resistance and is therefore the most preferred for the construction of gears.
TOOTH FORM
It is preferable to use straight teeth because the benefit of noise reduction given by helical teeth has no significance with plastic materials.
PRESSURE ANGLE
The most common angle for up to 17 teeth is 20°C; for gears with less than 17 teeth the pressure angle should be modified in a way to obtain tougher teeth.
PLAY
This is a very important factor for gears made of thermoplastic material, bearing in mind the expansion that occurs due to changes in operating temperature and the presence of moisture. Gears often have to work at temperatures which are higher than ambient, also for the frictional heating derived from the movement.
MATING GEAR MATERIAL
The best mating for gears made of plastic material is with one made of steel. The mating of two gears made of plastic material is unadvisable due to problems of heat dissipation from the crown gear, although it is possible with slow gear movement and good lubrication/cooling. When using two gears made of the same material the best solution is Arnite-Arnite. If the gear is mated with a steel gear, it is advisable that the surface finish on the teeth are as smooth as possible to avoid excessive wear and improve the life of the gear coupling.
Omnia Plastica technical
office is at your disposal for further explanations and for the solution
of your problems.
All data and informationare given in goods faith and are in no way binding
upon Omnia Plastica spa
The use of engineering plastics for the production of bearings has grown significantly due mainly to the good properties which are present in these materials:
Low coefficient of friction, even in poor operating conditions and without lubrication the friction coefficient is good. It can be further improved with the addition of lubrication either intermittently or continuously;
Self-lubrication, generally by a previous lubrication before the assembly, the bush made of engineering plastic can operate successfully in dry or dipped in water or other liquids;
Good mechanical features, the compression strength of the engineering plastic materials is from 500 up 1000 kg/cm2, and in addition to this, most of them have good impact damping.
Some materials have good dimensional stability even in presence of
various types of chemicals.
High wear resistance also without lubrication and for bearings which
operate in dusty environments.
CHOICE OF THE MATERIAL
If you want to design in the correct way a bearing made from a plastic material, the choice of the most suitable material must be done by bearing in mind the following variables:
The exact values of the above features can be found in our technical catalogue while below we summarize the main features of the engineering plastics most commonly used for the production of bearings:
PET - Arnite
It is one of the most suitable materials for the production of bearings and slideways, having a compression strength of about 850 kg/cm2, high superficial hardness, one of the lowest friction coefficients, good wear resistance and excellent dimensional stability both at temperature and at humidity, the latter being due to its low water absorption, Arnite also has very good mechanical features, such as the high compression strength and low deformation under weight. Its low thermal conductivity allows heat to be easily dispersed.
POM - Acetal
Acetal is a very good material for bearings, and is currently one of the most used due mainly to its good features vs cost ratio, the surface hardness and the fine performance in presence of a variety of chemicals. Its compression strength is 900 kg/cm2. Because of its easy machinability it is often the preferred for high production quantities. It is also suitable for use with foodstuffs.
PE 500 - Polyethylene 500 - 1000
The polyolefins that's to say either PE 500 or PE 1000 or other grades have a compression strength of about 300 - 400 kg/cm2 which is notably low if compared to the one of engineering plastics, but they are characterised by a high impact strength, and are therefore mostly used when a bush with high shock resistance without high loads is required.
PA6 ( Akulon 6 ) - PA 66 ( Akulon 66 )
The polyamides are materials mainly used in other applications as they aren't ideally suited for the production of bushes and bearings due to their bad dimensional stability which is caused by their high moisture absorption; they are however used for the good abrasion resistance under every conditions.
PA6 G - Omniamid G
CAST PA6 is irreplaceable for the production of large dimension bearings ( up diam. 1500 mm ) for heavy duty applications such as earth movers, cableway plants or big machines. For the best abrasion resistance, friction coefficient and dimensional stability choose either Omniamid G Oil, filled with special oil, or Omniamid G HR which has both graphite and oil fillers
PA 66.6 - Akulon GX
Polyamide copolymer, which maintains the same good abrasion resistance of extruded PA6 but has improved dimensional stability, higher surface hardness and compression strength. Due to its unique molecular structure, Akulon GX maintains its features even with ageing. It is also very easy to machine with machine tools. It's used for bearings where high wear resistance is required , such as on earth moving machinery, cableway plants etc.
PA11 - Rilsan
Polyamide copolymer, which maintains the same good abrasion resistance of extruded PA6 but has improved dimensional stability, higher surface hardness and compression strength. Due to its unique molecular structure, Akulon GX maintains its features even with ageing. It is also very easy to machine with machine tools. It is used for bearings where high wear resistance is required , such as on earth moving machinery, cableway plants etc.
PESU - Polyetheresulphone
One of the new super polymer materials having excellent high temperature
performance, and good dimensional stability at both high temperature
and in humid conditions. It is used for bearings which operate in highly
corrosive chemical environments.
PTFE ( Teflon )
PTFE can be used up 230 °C, it doesn't absorb water and has a low
coefficient of friction, but under pressure the material tends to deform,
because of molecular sliding. It has relatively poor mechanical features,
the deformation load being only 80 kg/cm2.
P.V.D.F
P.V.D.F. maintains the good chemical features of the fluoropolymers, while it has, compared to PTFE, higher mechanical resistances that's to say the load of the compression strength is about 500 kg/cm2. The maximum service temperature is 160 °C. and it maintains a low coefficient of friction. Its specific weight is lower than the one of PTFE thereof it is cheaper.
Shafts:
The mechanical shafts which work in contact with bearings made of plastic
material must be as tough as possible and highly polished. The normal
ground steel shafts can be used with good results, but better machining
will increase the life of the bearing. The use of shafts made of aluminium,
zinc or bronze without lubrication is not recommended because these
soft materials can present a rapid wear.
Lubrification:
The lubrication even if not necessary, increase the PV limit
according to the type and the quantity of lubrication. This fact reduces
the coefficient of friction, allows higher heat dispersion and removes
the fragments deriving from wear. It is important that the oil are not
acid or that they decompose in acid residuals at operating temperature.
Bearings which are not lubricated can provide good results as long as
the tip speed is not high. An improvement of the operating conditions
is obtained making some superficial grooves in a way that allows particles
resulting from the wear to be eliminated from the sliding area. The cooling
action of the lubricant, an air circulating system around the bearing
working as cooling will further increase the bearing life.
Thickness:
It's advisable to design bearings used for high speed movements with
relatively thin thickness, according with the constructive needs, in
order to facilitate the dissipation of the heat present in the surroundings
of the sliding surface. A thickness of about 20% of the diam. of the
shaft is in most cases very close to the optimal sizing. For slow movements
or with shock, increase the sizing.
Tolerances:
For the assembly of the bearing in its housing, there shouldn't be excessive
strains, it should have tolerances of a maximum of 0,1 mm. It's a good
rule to assemble the bearings with a good lubrication even if they
are planned for dry running.
Surface Pressure: It's the admitted load on the contact surface between shaft and bearing. Especially the maximum allowable superficial pressure ( kg/cm2) for the different materials are calculable by division of their compression strength from 3 to 5 referring to the kind of use of the bush. By determining the contact area between shaft and bearing it's possible to verify the allowable pressure for the bearing to be used in an optimum.
Temperature:
The operating temperatures of the bearings made of plastic materials
change in presence of lubrication and by different methods of assembly.
Especially the lubricating agent ( oil, lubricating oil ) operates
dispersing heatt from the bearing, thereby increasing its life. The
values of the features don't fall considerably to temperatures:
The maximum operating temperatures with low loads are more elevated and related to different factors.
Play:
The play of the bearing is essential in order to allow the thermal expansion
and the contraction. In order to avoid axial stretching the bushes
have to be produced with a slightly smaller length than its housing.
For the radial play of bushes made of Arnite (PET) and Acetal (POM)
that work with steel shafts use the following rule:
G= 0,012 mm + 0,025.S + 0,001.D where:
For the other bearings use a diametral play of 0,5 - 0,7 %.
Omnia Plastica technical office is at your disposal for further explanations and for the solution of your problems. News and data in this brochure are given only as simple informations without engagements.