In this article, linear axis are categorized according to their drive variants, with toothed belt drives, spindle drives, rack and pinion drives and linear motor drives being the four relevant types. However, you should note that the differentiation can vary, which means that they can be categorized not only according to their drive variants, but also according to the other distinguishing features listed in the table.
Differentiation of linear according to:
- Types of guide systems
- Internal or external guide systems
- Drive variants
- Base material
- Production of customized lengths or standard lengths
The differentiation by type of drive system is only one of five differentiation options for linear axis.
As it is not possible to make general statements about linear axis, the following section focuses on NTN linear axis of the SNR brand.
Toothed belt drives
Linear axis with toothed belt drive are suitable for fulfilling fast handling and positioning tasks. All axis are equipped with an AT or STD toothed belt, whereby the toothed belt clamping via the toothed segment in the toothed belt width enables unweakened clamping. Thanks to a radially adjustable bearing of the deflection pulley, the toothed belt tension can also be set precisely using the SNR toothed belt tensioning device, which consists of a force gauge and adapter devices for the respective axis type.
The advantages of this method of tensioning the toothed belt:
- No risk of overloading the toothed belt during installation
- No premature failure of the pulley bearing due to excessive belt tension
- Optimum running properties thanks to centered alignment of the toothed belt during installation
- low wear
Spindle drives
Linear axis with screw drives are characterized by the fact that they can be equipped with ball screws or trapezoidal screws. They are particularly suitable when there are high requirements for positioning accuracy and repeatability in combination with high rigidity of the drive element. In addition, direct drive adaptation is possible via a coupling and coupling cone or with the aid of a deflection belt drive. Spindle drives can also be used to achieve high positioning accuracy over long travel distances. In applications that require longer travel distances and higher speeds, spindle drives can be equipped with spindle support units. In addition to the fact that relatively high speeds are possible, these spindle support units also serve to shorten the free length of the spindles – i.e. the distance between the nut and the floating bearing – by half or two thirds, for example, and thus increase the critical speed.
Rack and pinion drives
Rack and pinion drives are another drive variant of linear axis. These are characterized by high operational reliability and are therefore suitable for vertical applications. As the rack consists of rack segments, theoretically unlimited travel distances are possible, which means that linear axis of any length can be built. For this reason, rack and pinion drives are ideal for use in large gantry axis, for example. The induction-hardened racks and gear wheels of this drive variant guarantee a long service life; it should also be mentioned that rack and pinion drives – even with heavy loads – have the highest rigidity in the drive system.
Linear motor drives
The use of linear motor drives is particularly suitable for applications where the highest demands are placed on positioning accuracy and repeatability. One major advantage is their flexibility, as they are suitable for both extremely slow (e.g. 1 cm per hour) and very fast movements. Linear motor drives are maintenance-free drive elements and can theoretically be used for unlimited travel distances. One disadvantage of linear motor drives is that they have no braking function in the power-off state, which can be critical in vertical applications.
Overview of the advantages and disadvantages of the drive systems
The following overview provides a direct comparison of the most important advantages and disadvantages of the drive systems mentioned above.
| Advantages | Disadvantages | |
| Toothed belt drive | • High dynamics • Long lengths • Cost-effective • Maintenance-free | • Lower dynamic operating load • Lower drive stiffness • Normally a gearbox is necessary • Lower repeatability |
| Spindle drive | • High positioning accuracy and repeatability • High feeding forces • High drive rigidity • Usually, no gearbox necessary | • Speeds limited by critical speed and DN value • shorter maximum lengths • Relubrication necessary |
| Rack and pinion drive | • Theoretically unlimited lengths possible • Several independently travelling drive units possible • High feeding forces • Highest drive rigidity | • Relubrication necessary • Lower positioning accuracy and repeatability |
| Linear motor drive | • Very high positioning accuracy and repeatability • Very high dynamics • Wear-free and maintenance-free drive • Theoretically unlimited lengths possible | • No breaking of the system in the power-off state (vertical use critical) • Relatively high costs |
It should always be made dependent on the respective application which drive variant is used.
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