Types of Guideways
The guideways are mainly categorized as per to the nature of friction among contacting surfaces of the operative element:
(1) Guideways with sliding friction
(2) Guideways with rolling friction
Guideways with Sliding Friction
The friction among the sliding surfaces is known as guideways with sliding friction. These guideways are as well called as slideways. The slideways are further categorized according to the lubrication at the interface of contacting surfaces. The friction among the sliding surfaces may be dry, semi-liquid, and liquid. While the lubrication is not present in between contacting surfaces, it is known as dry friction. Dry friction is rarely occurred in machine tools.
While two bodies slide with respect to each other having lubrication among them, the sliding body tends to rise or float because of hydrodynamic action of the lubricant film. The principle of slider is displayed in below Diagram.
Figure: Principle of a Slider
The hydrodynamic force,
Equation 1
Fh = C * vs
In which C is constant and depends on wedge angle θ, the geometry of sliding surfaces, viscosity of the lubricant and parameter of lubricant film.
vs stands for - sliding velocity.
W stands for - the sliding body.
The resultant normal force acting on sliding body,
R = Fh - W
From Eq. (1), it is clear that the hydrodynamic force increases along with increase in sliding velocity. The sliding body rests on the stationary body while hydrodynamic force is less as compared to the weight of the sliding body. Here, there are semi-liquid type friction conditions and within these types of conditions the two bodies are partially separated by the lubricant film and partially comprise metal to metal contact. The resultant normal force on sliding body starts to perform upwards and the body floats like hydrodynamic force is greater as compared to the sliding weight of the body. The sliding surfaces are totally separated by the lubricant film and liquid friction takes place at their interface. The slideways where the sliding surfaces are separated by the permanent lubricant layer are termed as hydrodynamic slideways. This permanent lubrication layer is because of hydrodynamic action. A permanent lubricant layer among the sliding surfaces can be obtained by pumping the liquid into the interface under pressure at low sliding speed. The sliding body is lifted through this permanent lubricant layer. Such slideways are termed as hydrostatic slideways.
Guideways with Rolling Friction
These are also known as anti friction ways. The anti friction slideways may be categorized as per to the shape of the rolling element as:
(a) Roller type anti friction ways by using cylindrical rollers.
(b) Ball type anti friction ways by using spherical balls.