Hey cold, you don’t scare me!
You know when you leave your car sitting outside in the cold in winter and, when you have to use it, you need to warm it up first? Well, now imagine the same effect on the various components of your project. Low temperatures modify the physical and mechanical-dynamic characteristics of the materials and can cause premature malfunctions of the mechanical devices housed within.
The design of a mechanical device must always take into consideration the lower limit of operating temperature to which the various components can go. In these cases, the lubricants have the task of guaranteeing operation up to such temperatures. But which lubricants? Oils or greases? In this post, we will focus on the appropriate use of low temperature greases.
Low temperature, effects on mechanical devices: a vicious circle
Let's start by trying to understand why lubrication is important for low temperatures. The decay process caused by the inadequacy of the "cold" lubricating fluid is rather linear and characterised by a precise sequence of events:
- Increased internal friction
resulting from the aggregation of polymers at low temperatures. It produces an increase in the friction coefficient of the surfaces in mutual sliding, due to the greater force required to slide the fluid layers of the lubricating film. - Increased power absorption
due to the higher energy delivery required to make the device work properly - Increased mechanical wear
since greater friction generates an increase in contact and rubbing wear, with loss of material and compromised integrity of the device - Reliability reduction
given that the number of operating cycles originally envisaged in the design is no longer guaranteed. - Device lock
when the device no longer responds to commands or begins to show more and more frequent defects - Loss of system functionality
when the device is an integral part of a more complex system and its lack of functionality prevents the whole system from performing its function.
The lubricant is thus the solution to seek out in order to manage low operating temperatures. But which lubricant is able to best meet these needs? Oil or grease?
Why use a low temperature grease instead of an oil
In general, grease is preferred over oil in all fields of application characterised by an open housing to be lubricated – think valves, gearmotors or bearings. In all such components, a low-viscosity lubricant could leak out so a high adhesiveness is required to favor the position in the housing.
On the other hand, a high viscosity could be counterproductive at low temperatures, due to the greater friction that would be created. This is the case, for example, with cryogenic valves, gearmotors or bearings that operate in cold external environments or air-conditioned internal places.
How then can a lubricating grease be improved or optimized to be compatible with low temperature use?
Grease for low temperatures: the importance of the base oil
The components that make up a lubricating fluid are essentially four:
- Base oil
- Thick or consistency agent
- Solid lubricants
- Additives
Among these, the base oil is certainly the component present in the greatest quantity, as well as the one that affects the performance at low temperatures the most. What must be correctly leveled is the so-called pour point, which is the minimum temperature that allows a lubricant to maintain its sliding properties within certain values.
In this sense, a synthetic base oil is far more suitable as it maintains its smoothness properties at lower temperatures than a mineral oil and allows to compensate for the excessive adhesiveness / viscosity of the greasy lubricant compared to oil.
Read more from our blog: Synthetic oil and mineral oil: 5 characteristics compared.
