Precision rail guides

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Accuracy of adjacent components
An important criterion for the correct
performance of a rail guide system is
the accuracy of the associated components.
The higher the demands for
accuracy of guidance and for smooth,
easy operation, the greater the attention
which must be paid to the
accuracy of form and position of the
associated components. Generally
the same accuracy requirements
should be applied to these components
as to the rail guides themselves.
The adjacent table shows the
values, for each tolerance class, of
the surface roughness, perpendicularity
and parallelism of the adjacent
components.
To assure an even load distribution
over the roller length, the maximum
difference in height of the supports
for a rail guide should not
exceed
∆h = 0,1 · B 1
where
∆h = maximum height deviation, µm
B1 = mean distance between two
rail guides, mm
To obtain good support for the
rails on the associated components,
the attachment holes should be carefully
deburred (Figs. 9 and 10).
Selection of rail guides
When selecting a rail guide, the
length of travel, load carrying capacity,
requisite life and stiffness are the
most decisive factors. Other important
parameters include the requisite
speed of travel, lubrication, operating
temperatures, ease of movement,
environmental influences and certain
design constraints, for instance
whether “clamped” or “floating” guidance
is required. If further information
is needed, please contact SKF.
The selection of the size and length
of the rolling element assemblies is
mainly determined by the required
Fig. 9
Fig. 10
Accuracy of form of support surfaces
Characteristic Symbol for Permissible
deviation of form
Charac- Tolerance Dimensions Tolerance class
teristic zone
P10 P5 P2
Roughness R a a µm 1,6 0,8 0,2
Perpendicularity t 1 /t 2 µm/mm 0,3 0,3 0,3
Parallelism §§ t 3 /T 4 µm depending on the
guide length L (mm)
3 2 1 200
6 4 2 500
10 6 3 1 000
load carrying capacity, life and stiffness.
For light, centrally acting loads
and moderate demands for speed of
travel, it is possible to use practically
all the types of guides listed in this
catalogue. However, technical and
economic reasons often dictate the
choice of the most appropriate model
for a given application.
The length of a rail guide and of
the individual rails is mainly determined
by the travel as well as the
length of the rolling element assemblies,
which depends on the requisite
load carrying capacity and life.
The following relationships serve
as guidelines for determining the
length of rail guides and rolling element
assemblies:
For a given stroke:
Cage length = stroke, at least
For a given cage length:
Rail length = cage length
+ 0,5 x stroke
For a given rail length and stroke, the
cage length is is obtained from:
Cage length = Rail length
– 0,5 x stroke
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