GEARS of METALON and POLYACETAL

Engineering thermoplastics cannot be used for small-sized power gears because of their rather low mechanical strength in comparison with metal gears. Consequently, they are used for those applications where this strength limitation in small sizes is compensated for by other advantages such as:

      -          Smooth and quiet operation
-          Corrosion resistance
-          Wear and abrasion resistance (also when unlubricated and in wet or dusty surroundings)
-          Impact resistance
-          Economy in gear production
-          Low weight. 

LOAD CAPACITY OF SPUR GEARS
Experience has shown that, generally, the following calculation method, based on the well known formula of Lewis, can be successfully used:
Allowable power to be transmitted: KW P = MxWxPxFxS
                                                                                        6000000
WHERE: S= allowable stress (N/mm²) M = module (mm) W= tooth width (mm) P=pitch circle diameter (mm) Z=number of teeth N=speed (rpm) f1= speed factor 0.75   +0.25
                                                                        1+V

V=circumferential speed of the gear at the pitch circle = ÕxPxN  (m/sec)
                                                                                                     60000

ALLOWABLE STRESS (N/mm²) for a combination METALON / steel operating in normal surroundings = S

REMARKS

The calculations method & figures given above are highly conservative & such that the calculated allowable power can be transmitted by the gear as full load over a long period of time without failure. When calculating, one should consider however, that in practice gears are rarely operating under full load and also that the actual power transmitted by the gear is mostly lower than the given power of the drive. Since it is difficult to translate the operation of most machines into correct mathematical data. It will often be a practical test that will assess the application possibility of an METALON Engineering plastics gear.

HELICAL & BEVEL GEARS

Metal helical gears are sometimes used in order to reduce the noise level. This more quiet operation can also be obtained by using cheaper METALON spur gears because of their very good mechanical damping properties. Obviously, the load capacity calculation method given above for spur gears can, in a modified form, also be used for the calculation of helical and bevel gear. 

WORM GEARS

METALON worm gear systems are difficult to calculate and are restricted to applications involving low loads and speeds.

DESIGN GUIDE FOR GEARS

1) CLEARANCE

Due to the much bigger thermal expansion and/or the expansion cased by moisture absorption as well as the radius of curvature of the corner where the tooth joins the root circle (fillet radius-= 0.2 x module) initial clearance required is greater than for metal gears. Head Clearance = 0.2xmodule & backlash 0.05 x module is normally applied.

Gears with fixed center distance (fixed shafts) The outside diameter of the gear blank has to be machined to a dimension equal to the theoretical outside circle diameter less a value that can be taken from above as a function of operating conditions. The teeth have then to be HOBBED to their theoretical depth. In case of adjustable center distance shaft can be taken further apart by the same amount as calculated.

2) CHOICE OF MATING GEAR MATERIAL
a)      Combination plastics/metal
This is mostly the recommended combination because of the good heat dissipation possibilities and the relatively small tooth deformation. Just as for bearings, the wear of both meshing parts decreases with increasing hardness of the metal counter faces. The teeth of the metal gears must have a good finish but don’t need to be ground.

b)   Combination plastics/plastics (avoid as far as possible)
This combination yields worse heat evacuation possibilities which leads to bigger tooth deformations than the above mentioned one. Consequently, it can only be used for the transmission of smaller powers. The combinations METALON & POLYACETAL are recommended
3) LUBRICATION.
Although METALON gears can work excellently without lubrication a longer life can be expected and higher powers can be transmitted by lubricating. Anyway, an initial grease-lubrication is always recommended, in order to facilitate the running in even when no further lubrication will be applied.
4) ASSEMBLY
The forces are usually transmitted from the shaft to the gear by means of one or more parallel keys. Usually METALON gears have in addition to be press fitted on to the shaft Power transmission by using press fits only is rarely suitable. When operating temperatures are above 50°C it is good idea to use metallic boss with key slot & METALON gear fixed with Bolts to the boss. (this also saves costly METALON) As boss thickness is generally 2 /3 times the tooth width.

      5) RING GEARS ON METAL CORES
       For very large diameter (300mm & above upto 2000 mm) METALON gears are normally assembled on metal
       cores by  means of shrink fitting additionally provided with parallel keys & or bolts. For calculating clearances 
       of large gears
        consult us.

                                                                                            REMARK S

  -     For METALON gears, it is recommended to provide keyways with rounded corners (r>0.5mm). This in order to avoid stress concentrations.
A minimum distance of 2.5 x module should be provide between the root circle of the gear and the upper surface of keyway.

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