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Manual locating units – Precision in small spaces
Locating units play a central role in mechanics and are essential for designers in many different applications. In this article, we will review how locating units, their different types and their applications work.
What are manual locating units?
A locating unit, positioning stage or table is a component that serves to bring a workpiece or tool into a specific position. The positioning stage can be aligned horizontally or vertically and is generally mounted with its main body on a stable base plate. The position of the moving table can be changed by turning a crank or adjusting screw. Alternatively, the position can be adjusted via a threaded rod or a spindle drive.
The main function of locating units is to perform a precise movement of workpieces or tools in a specific direction or around an axis. Here, various guide systems are used, via which linear movement is implemented in a specific direction or precise rotary movement about an axis.
Multi-axis systems combine multiple types of motion to perform more complex motion sequences. Depending on the design of the locating unit, high precision, low construction height or large stroke lengths may be the focus of the design.
Types of locating units
There are many different types of locating units that are suitable for various applications. The most common types with their respective properties are listed below. Locating units are mechanical units with guides, feed mechanisms and snaps.
Stages consist of a base body, which is fastened to a surface within the mechanical system, and a movable table on which workpieces or tools can be fastened. By means of an adjustment mechanism, the movable table is moved against the base body either linearly or rotationally. By combining positioning stages with different linear- and rotational axes, three-dimensional positioning- and adjustment processes along the X -, Y- and Z -axis as well as rotational movements with a high degree of precision can be carried out.
Linear tables
Linear tables represent the simplest and most commonly used type of positioning. The moving table moves along a fixed axis during linear motion. The position of the table is determined using an adjusting screw.
Rotation tables
With rotation tables, the movable part of the positioning table is rotated against the base body in order to enable a rotation of the workpiece or tool attached to it. Angular adjustment up to 0.5° can be made.
Goniometer tables
Goniometer tables have an arc-shaped contact surface between the main body and the movable part of the positioning table. As a result, the rotation of the workpiece mounted on the stage is made possible by an axis that lies above the locating unit. Due to the displacement of the rotating axis, the radius of movement of the goniometer table covers only a small angle range, but very precise adjustment processes up to angular differences of 0.1° can be realized as a result.
How is the position of locating units set and fixed?
When selecting the appropriate management system, the requirements resulting from the planned application must be considered in detail. In addition to load-carrying capacity and tolerances with regard to straightness, inclination and parallelism, the characteristics of the guide, position and clamping systems should be subjected to a more precise test.
Guide systems
Depending on the requirements for smooth running, precision, load capacity and speed when guiding the moving tables, different linear guide profiles can be used. The most common linear guide profiles include dovetail guides, cross roller guides and ball guides.
| Dovetail rail guide | Cross roller guides | Ball guides | |
|---|---|---|---|
| Structure | A sliding trapezoidal profile groove or profile facilitates the guide. | Enclosed cylindrical rollers are positioned alternately crosswise and lie between two groove rails. The roller movement has an effect on good guide qualities. | Steel balls run in gothic arched shaped grooves, which are incorporated into the table profiles. The roller movement has an effect on good guide qualities. |
| Straightness | Standard: 50 μ High precision: 30 μ |
Standard: 50 μ High precision: 3 μ |
High precision: 1 μ Engine gearboxes |
Control mechanisms
There are various control mechanisms available for adjusting the position for the respective types of guide systems on stages. The selection of the control mechanism has a direct effect on the precision and travel distance, which can be achieved by turning the adjusting screw.
| with pinion drive | Adjustment screw | Adjustment screw | Micrometer screw | Micrometer screw (rough/fine adjustment) | Digital micrometer screw | |
|---|---|---|---|---|---|---|
| Guide mechanism | Dovetail guide | Dovetail guide | Cross roller/linear ball guide | Cross roller/linear ball guide | Cross roller/linear ball guide | Cross roller/linear ball guide |
| Travel per revolution | 17 - 20 mm | 0.5 - 10 mm | 0.5 - 1 mm | 0.5 mm | 0.025 - 0.5 mm | 0.5 mm |
| Suitable for fast feed | ✓ | ❌ | ❌ | ❌ | ❌ | ❌ |
| Suitable for fine feed | ❌ | ✓ | ✓ | ✓ | ✓ ✓ | ✓ ✓ |
| Suitable for precise positioning | ❌ | ❌ | ✓ | ✓ | ✓ ✓ | ✓ ✓ |
| Special features | Screw pitch freely selectable | Less expensive than micrometer screw | Precision positioning in increments of 0.01 mm | Precision positioning in increments of 0.5 μ Allows for a finer feed than conventional micrometer screws |
Precision positioning in increments of 0.1 μ With digital display Allows for a finer feed than conventional micrometer screws |
Detection mechanisms
The locating units are either secured by means of screws, locks or lever clamps. While screws and locks prevent the movement of the table by a connection between the main body and the movable table, lever clamps fix the control mechanism itself.
| Standard clamping device | Writing lock | Opposite clamp | Slotted clamp | Lifting clamp | |
|---|---|---|---|---|---|
| Properties | The clamping plate is pressed laterally against the table by a clamping screw. This is the cost-effective standard solution. | The table is rendered immovable by locking a disc. The table surface remains unstressed. Position changes are prevented. | On the opposite side of the micrometer screw, the carrier is secured with a screw. For greater vibration resistance and even more holding power, the screw is secured with a nut. | The adjusting handle shaft is clamped directly. Compared to the conventional design, this solution achieves a greater retention force. | The final clamping effect of the clamping screw is achieved by means of an easily operated lever. |
Locating unit applications
The applications of locating units are diverse and exist wherever workpieces or tools must be precisely aligned. Some examples of the use of locating units are:
- Positioning of sensors to detect errors during labeling
- Positioning of instruments for leakage testing on bottles or other containers
- Positioning of temperature sensors
- Positioning of rolls for applying adhesive to a workpiece
Factors in the selection and integration of locating units
Some factors must be considered when selecting and integrating locating units. These include:
- Accuracy: The accuracy of a locating unit is a decisive factor in the selection. Depending on the application, locating units can be positioned with low, medium and high precision.
- Travel: The travel distance of a locating unit describes the range of motion that can be covered by the positioning unit.
- Load-carrying capacity: The load-carrying capacity indicates how much load (measured in Newtons) the locating unit can carry. Depending on the application, locating units with high rigidity can also be used, which are particularly suitable for loads whose center of gravity is not in the middle of the table.
- Compatibility: The compatibility of a locating unit with other components is an important factor. In doing so, attention should be paid to drilling patterns, size, weight and the materials used.
- Environment: The application environment determines, for example, requirements with regard to the type of protection or the temperature resistance that are placed on the locating unit.
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