Greases and lubricating oils for geared motors
The right compromise between speed and loads

Gearmotors represent one of the most complex cases of mechanical lubrication, needing a lubricant capable of ensuring the correct separation of friction surfaces in a context of variable dynamic along the chain of motion transfer where:

• The initial stages are characterised by high rotational speeds, low torques and low dynamic loads, where the lubricant tends to be centrifuged and moved away from the contact zones – at the same time, it must offer minimal viscous resistance to limit energy absorption
  
  
• In the later stages, there is a progressively decreasing speed with loads having increasing impact, where the lubricating film tends to be "broken", threatening direct contact between surfaces with the consequent risk of mechanical wear

A geared motor is thus a heterogeneous tribological system. At different points in the transmission (as seen in the 5-stage model shown above), very different operating conditions occur. Adequate lubrication must be able to intercept such at their maximum and minimum functioning, with:

• The magnitude of mechanical impact loads
• Maximum and minimum gear rotation speeds
• The minimum and maximum operating temperature
• The composition of friction materials (plastics and/or metals)
• The dominant type of friction

The main threat of irregular gearbox operation, along with mechanical wear, is the rise in temperature often caused by inadequate lubrication.

Both types of friction coexist in a gearbox: sliding and rolling. In instances of dominant sliding friction, such as in helical-worm wheel couplings, where a "rapid" reduction in rotational speed in favour of transmitted torque is required, the lubricant composition must be balanced with the use of specific components:

• Synthetic high-performance base oil
• EP additives – extreme pressure
• AW additives – anti-wear
• Solid lubricants – PTFE, MoS₂, graphite, boron nitride and so on

The choice of lubricant consistency is mainly related to the application context and the need to remove heat during operation in order to avoid:

• Dangerous expansion of materials
• Misalignment of rotation axes, and
• The consequent triggering of mechanical wear

The choice of oil or grease solution to be used affects both the design of the containment chamber and the method of lubrication.

In the case of oil lubrication 

• The lubrication chamber must be perfectly sealed and the gears immersed to an ideal level for the formation of the lubrication film during the various rotation speeds
  
  
• The lip seals and O-rings assembled on the rotating shafts must ensure a perfect seal so as to prevent oil from escaping from the containment chamber and securing the rotating bodies
  
  
• Periodic monitoring of the residual state of the oil is recommended in order to assess the residual lubricating power and foresee topping-up and/or replacement cycles in order to prevent critical operations

In the case of grease lubrication: 

• It is important to apply evenly on the tooth flanks to ensure its immediate distribution on the friction surfaces
  
  
• It is necessary to balance the lubricant's adhesiveness and viscosity to prevent excessive energy absorption or centrifugation
  
  
• A product with minimal oil separation is preferred due to the lack of sealing elements in the gear chamber