When two or more linear guideways support the same moving table, the rails must guide the table along a compatible path. If the rails are not parallel, the guide blocks may be forced sideways or twisted as the table moves. The system may still operate, but internal load, friction and wear can increase significantly.
There is no single parallelism value that applies to every linear guideway installation. The acceptable deviation depends on the guideway series, rail size, preload, accuracy grade, rail spacing, stroke length, mounting method and required machine accuracy. The correct requirement should therefore come from the guideway specification, the machine drawing and the actual operating conditions.
What Does Parallelism Mean in a Multiple-Rail System?
In a typical two-rail installation, one rail is treated as the master rail. Its datum surface establishes the direction of travel. The second rail is the secondary rail and must be aligned with the movement defined by the master rail.
Parallelism should be checked in more than one direction. Lateral parallelism describes whether the secondary rail moves toward or away from the master rail along the stroke. Vertical parallelism describes the change in height between the two rails. Both conditions can affect the guide blocks and the moving table.
| Condition | What It Describes | Possible Effect |
|---|---|---|
| Lateral parallelism | Change in the horizontal distance between the rails along the stroke | Side loading, binding and uneven running resistance |
| Vertical parallelism | Change in relative rail height along the stroke | Table twist, uneven block loading and reduced motion accuracy |
| Rail-spacing error | Incorrect or changing distance between two rails | Internal stress after the table is tightened to the blocks |
Is There a Standard Parallelism Tolerance?
A universal value should not be used for all installations. A normal-clearance guideway used in general automation may tolerate more mounting variation than a preloaded guideway used in a precision machine tool. Smaller guideways, higher preload and higher accuracy requirements usually make the system more sensitive to alignment error.
Rail length also matters. A deviation stated over the full travel is not the same as a local high point caused by a burr or chip. A long and gradual change may affect the system differently from a sharp error concentrated near one mounting bolt.
The permissible parallelism deviation should be confirmed from the technical data for the selected series and size. The machine designer must also consider whether the required positioning and geometric accuracy of the complete axis is stricter than the guideway manufacturer's basic installation allowance.
Factors That Determine the Required Parallelism
Preload is one of the most important factors. Preloaded blocks provide greater rigidity, but they have less internal freedom to absorb mounting error. If parallel rails are misaligned, preload can combine with installation stress and produce excessive running resistance.
Rail spacing and table rigidity also influence the result. A wide rail spacing can improve resistance to moment loads, but height error between the rails may twist a rigid table. A thin or flexible table may deform during bolt tightening and hide the alignment problem temporarily.
Operating conditions must also be considered. High speed, rapid acceleration, heavy loads and frequent reversing make uneven internal loading more serious. Precision measuring equipment and machine tools normally require tighter control than low-speed transfer equipment.
How to Measure Parallelism Between Linear Guideways
Before measurement, clean the rail datum surfaces, mounting shoulders and machine base. Remove burrs, paint, rust-preventive residue and metal chips. Even a small particle trapped under a rail can produce a local height or alignment error after the bolts are tightened.
Install and align the master rail first. Its reference side should be seated against the machine datum or aligned with the specified reference method. Once the master rail is secured, use the blocks on that rail to carry a dial test indicator along the stroke.
To check lateral parallelism, position the indicator against the appropriate datum side of the secondary rail. Move the carriage slowly from one end to the other and record the maximum and minimum readings. Their difference represents the measured lateral variation over the inspected length.
Vertical variation can be checked against the top reference surface of the secondary rail or against a suitable precision reference connected to the secondary blocks. The measuring setup must be rigid enough that indicator-stand movement does not become part of the reading.
For a detailed mounting sequence, see How to Align a Linear Guideway Accurately.
Why the Moving Table Must Be Checked
Measuring the rails alone is not always enough. The guide blocks, table mounting surfaces and bolt-tightening sequence can introduce additional stress. After the rails are aligned, place the blocks in their intended positions and attach the moving table without forcing it into place.
Tighten the table bolts gradually and in a controlled sequence. Move the table through the full stroke after each tightening stage. A noticeable increase in resistance may indicate rail-spacing error, unequal rail height, a distorted table or incompatible block mounting surfaces.
Running smoothness is a useful installation check, but it does not replace dimensional measurement. A lightly preloaded or clearance-type guideway may move smoothly even when the alignment does not meet the machine's accuracy requirement.
Common Causes of Parallelism Error
- Burrs, chips or dents on the mounting surfaces
- An inaccurate or interrupted reference shoulder
- Uneven rail-mounting surfaces
- Fully tightening the secondary rail before alignment
- Using the moving table to force misaligned blocks into position
- Incorrect block spacing or table-hole position
- Rail movement during final bolt tightening
- Machine-base deformation after assembly or loading
What Happens When Parallel Rails Are Misaligned?
Misalignment introduces forces that are not part of the intended external load. These forces act on the balls or rollers, raceways, seals, block bodies and mounting bolts. The result may be uneven movement, abnormal noise, local temperature rise or an increase in drive-motor current.
Long-term operation under installation stress can reduce service life and cause uneven wear between the blocks. It can also affect the straightness, positioning stability and repeatability of the complete axis. Increasing lubrication or selecting a higher preload will not correct a geometric alignment error.
Practical Acceptance Checklist
Before accepting a multiple-rail installation, confirm that the measured lateral and vertical deviations are within the limits specified for the selected guideway and machine. Recheck the readings after all rail and table bolts have reached their required tightening torque.
Move the table through the entire usable stroke and check for binding, changes in resistance, unusual noise and unwanted table movement. For a driven axis, low-speed testing can also be used to observe motor current and running consistency, but this should support rather than replace geometric inspection.
If the required tolerance is not available, do not select an arbitrary value from another guideway size or machine. Confirm the guideway series, size, preload, accuracy grade, rail length, rail spacing and application requirements with the supplier.
Conclusion
Multiple linear guideways must be parallel enough to share the load without forcing the blocks or moving table into incompatible paths. The requirement is not one fixed number for every system. It must be selected according to the guideway specification and verified in both the lateral and vertical directions after final tightening.
DLY supplies linear guideways for machine tools and industrial automation equipment. When requesting technical confirmation, provide the rail model, size, preload, accuracy grade, rail length, rail spacing, load and installation drawing.
Contact DLY Linear Motion Team
Zhejiang DLY Automation Manufacturing Co., Ltd.
Email: export@dlybearing.com
