Designers often treat linear rail sizes as simple width numbers: 15, 20, 25, 30, 35, and so on. However, that shortcut only identifies a product family; it cannot complete an engineering selection. Moreover, the nominal number does not always equal the measured rail width. Two guides with the same nominal size can also have very different assembly heights, block lengths, load ratings, and moment capacity.
Therefore, start with the machine rather than a familiar catalogue number. First, determine how the machine applies the load and where the center of gravity sits. Next, confirm how many rails and blocks will share the load, how much installation space the machine provides, and how much rigidity the axis needs. If you are new to the component, review how linear guides work and what their main parts do before using this size-selection guide.
1. What Does a Linear Rail Size Actually Mean?
Read the Size Number as a Family Code
A nominal size identifies a compatible rail-and-block profile family. In practice, it groups mounting dimensions, block options, rail-hole patterns, and catalogue load ratings. Therefore, use the number to narrow the product family, but do not treat it as a complete dimensional description.
Use Exact Catalogue Dimensions for Machining
For example, an IMTEK TOH25 rail belongs to the nominal size 25 family, while the catalogue lists an actual rail width WR of 22 mm. In addition, a TOH25 block creates a 40 mm assembly height. Therefore, machine the base pocket, shoulder, and bolt pattern from the exact drawing. Otherwise, an assumption that “size 25” means a 25 mm-wide rail can create a replacement or mounting conflict.
| Catalogue term | What it describes | Why you must check it |
|---|---|---|
| Nominal size | The product-family designation, such as 15, 20, 25, or 35 | Useful for initial classification, but not sufficient for machining or replacement |
| Rail width WR | The actual width of the steel rail | Determines pocket width, shoulder position, and available mounting space |
| Assembly height H | The distance from the rail base to the block mounting surface | Affects axis stack height, table position, and drive alignment |
| Block width and length | The external carriage footprint | Controls mounting-hole layout, payload interface, moment resistance, and moving envelope |
| Dynamic load rating C | A catalogue value used in rated-life evaluation under rolling motion | Compare it with the equivalent working load and the rating basis for that series |
| Static load rating C0 | The load-related limit used to evaluate permanent raceway deformation | Important for stationary load, shock, vibration, emergency stops, and low-speed axes |
| Allowable static moment | The block’s resistance to pitch, yaw, and roll moments | Often becomes the controlling value when the load is offset from the block center |
Video: Choosing Guide Size from Installation Space
This IMTEK TechTalk focuses on guide size and installation space. In addition, it replaces the previous rail-length video, so the visual content now supports the same search intent as the article.
First, use the video to screen out profiles that do not fit the available width or height. However, do not use it as the final calculation. Next, check the remaining options for load, moment, rigidity, and safety.
2. Select Linear Rail Size from Load, Moment, and System Layout
The total payload only starts the calculation. For example, four well-spaced blocks can support a 200 kg table differently from one short block under a 200 kg cantilever. Moreover, acceleration, cutting force, cable drag, tool reaction, and center-of-gravity position can change the load on each block.
Collect the Project Data Before Opening the Size Table
| Project input | Questions to answer | How it affects rail size |
|---|---|---|
| Payload and moving mass | What moves with the table, fixture, tooling, cables, and drive components? | Sets the basic vertical and inertial load level |
| External forces | Are there cutting forces, pressing forces, belt tension, robot reactions, or process loads? | May control the size even when the payload itself is light |
| Acceleration and deceleration | What are the normal and emergency-stop values? | Changes inertial load and moment during starts, stops, and reversals |
| Center-of-gravity offsets | How far is the load from the block centerline in each direction? | Creates pitch, yaw, and roll moments |
| Rail and block arrangement | One rail or two? One, two, or four blocks? What are the rail and block spacings? | Controls how the layout shares load and moment |
| Mounting orientation | Horizontal, vertical, wall-mounted, inverted, or inclined? | Changes load direction and safety considerations |
| Shock and vibration | Will the axis see impact, tool chatter, floor vibration, or repeated end-stop contact? | Raises the required static safety margin and may favor a stiffer guide type |
| Available space | What are the maximum assembly height, rail width, block width, and table thickness? | May require a low-profile, miniature, square, or flange configuration |
Check the Load on Each Block, Not Only the Total Payload
After you define the layout, calculate the equivalent load on the most heavily loaded block. However, do not simply divide the payload by four when the center of gravity sits off-center or external forces act outside the rail spacing.
Use the following basic moment relationship:
For example, a modest force can create a demanding moment when the lever arm is long. Therefore, a tall bracket, wide overhung table, robot arm, or side-mounted actuator may need a longer block, wider rail spacing, or a dual-rail arrangement before it needs a larger nominal size. When the center of gravity sits away from the guide plane, use the detailed guide for linear guide moment-load calculation.
After you calculate the block loads, compare the highest value with the model’s basic static load rating, basic dynamic load rating, and allowable static moments. Meanwhile, keep the rating purposes separate because static and dynamic load ratings answer different engineering questions.
Use Geometry Before Automatically Upsizing
Increasing rail size can add capacity. However, it is not the only solution. Instead, improve the support geometry when the machine layout allows it:
- Increasing the distance between two blocks improves resistance to pitch or yaw moments.
- Increasing the distance between two parallel rails improves resistance to roll moments.
- Using a long block can increase load and moment capacity without changing the rail profile.
- Moving the payload closer to the guide plane reduces the moment at its source.
- Adding a second rail may be more effective than replacing one small rail with one very large rail.
Therefore, define block quantity and center spacing before you release the final profile. Then review choosing the number and arrangement of linear guide blocks when the load position or moment demand makes the layout unclear.
3. Check Actual Dimensions and Block Geometry Before Choosing a Larger Size
Catalogue data makes the size decision more concrete. First, use one consistent block family for comparison. The table below uses the IMTEK TOH-CA standard square block to show how rail width, assembly height, block length, and load-related values change as nominal size increases. However, do not apply this table to every block style or series.
| Model | Assembly height H | Rail width WR | Block length L | Dynamic rating C | Static rating C0 | Static pitch/yaw moment |
|---|---|---|---|---|---|---|
| TOH15CA | 28 mm | 15 mm | 61.4 mm | 14.7 kN | 23.47 kN | 0.10 kN·m |
| TOH20CA | 30 mm | 20 mm | 77.5 mm | 27.1 kN | 36.68 kN | 0.20 kN·m |
| TOH25CA | 40 mm | 22 mm | 84.0 mm | 34.9 kN | 52.82 kN | 0.33 kN·m |
| TOH30CA | 45 mm | 28 mm | 97.4 mm | 48.5 kN | 71.87 kN | 0.53 kN·m |
| TOH35CA | 55 mm | 34 mm | 112.4 mm | 64.6 kN | 93.88 kN | 0.81 kN·m |
| TOH45CA | 70 mm | 45 mm | 139.4 mm | 103.8 kN | 146.71 kN | 1.55 kN·m |
| TOH55CA | 80 mm | 53 mm | 166.7 mm | 125.3 kN | 211.23 kN | 2.64 kN·m |
| TOH65CA | 90 mm | 63 mm | 200.2 mm | 213.2 kN | 287.48 kN | 4.27 kN·m |
Technical-data note: the values above apply to the listed IMTEK TOH-CA standard square blocks. Therefore, use the relevant catalogue table for other block shapes, long blocks, guide series, preload classes, and accuracy classes.
What the TOH Data Tells You
First, nominal size does not always equal the measured rail width. TOH25, TOH30, TOH35, TOH55, and TOH65 use rail widths of 22, 28, 34, 53, and 63 mm respectively. Therefore, machine from the dimensional drawing rather than the product name.
Second, capacity does not rise in a simple one-to-one ratio with nominal size. Moving from size 20 to size 25 changes assembly height, block footprint, mounting pattern, load ratings, and moment capacity. Therefore, ask “Which model passes the load and moment checks inside the available machine envelope?” rather than “How many millimeters larger should the rail be?”
Third, a long block may solve the problem without a larger rail profile. The following comparison shows how block geometry changes the available capacity within nominal size 25.
| Size 25 option | Guide type | Assembly height | Rail width | Block length | Dynamic rating C | Static rating C0 | Pitch/yaw moment |
|---|---|---|---|---|---|---|---|
| TOH25CA | Heavy-load ball guide, standard block | 40 mm | 22 mm | 84.0 mm | 34.9 kN | 52.82 kN | 0.33 kN·m |
| TOH25HA | Heavy-load ball guide, long block | 40 mm | 22 mm | 104.6 mm | 42.2 kN | 69.07 kN | 0.57 kN·m |
| TTH25CA | Low-profile ball guide | 33 mm | 22 mm | 82.6 mm | 16.27 kN | 32.40 kN | 0.32 kN·m |
For example, TOH25HA offers higher listed load and moment values than TOH25CA while it keeps the same 40 mm assembly height and 22 mm rail width. Therefore, it can help when the machine provides more longitudinal block space but cannot accept a larger profile. By contrast, TTH25CA lowers assembly height to 33 mm, yet it does not provide the same capacity as TOH25CA.
Ball and Roller Guides Can Prioritize Different Performance
Moreover, nominal size does not describe the rolling-element design. The following size 35 example compares a ball-guide block with a roller-guide block at the same 55 mm assembly height and 34 mm rail width.
| Size 35 option | Rolling element | Assembly height | Rail width | Block length | Static rating C0 | Pitch/yaw moment |
|---|---|---|---|---|---|---|
| TOH35CA | Ball | 55 mm | 34 mm | 112.4 mm | 93.88 kN | 0.81 kN·m |
| TZH35CA | Roller | 55 mm | 34 mm | 124.0 mm | 105.2 kN | 1.44 kN·m |
In this comparison, the roller block is longer and lists a higher static rating and pitch/yaw moment capacity. Therefore, engineers often choose a roller guide when stiffness and moment resistance dominate the design. However, compare dynamic ratings across guide types only after you check each catalogue’s rated-life definition and test basis.
4. Match the Rail Size to the Guide Series and Application
After you establish the required capacity and installation envelope, choose the guide architecture that best fits the axis. However, an application label such as “CNC,” “robot,” or “semiconductor equipment” cannot determine the final size by itself. Instead, use the application to narrow the series before detailed calculation.
| IMTEK series | Design emphasis | Typical selection conditions | Main size checks |
|---|---|---|---|
| TO ball-type guides | Balanced load capacity, smooth motion, and broad block options | General automation, handling, packaging, machine tools, transport devices, and standard industrial axes | Equivalent block load, moment, standard vs. long block, square vs. flange mounting |
| TT low-profile guides | Reduced assembly height for space-limited equipment | Compact automation, semiconductor equipment, inspection systems, and machines with restricted vertical space | Available height, reduced capacity compared with heavier profiles, block mounting interface |
| TF cover-strip ball guides | Rail-top protection with a cover-strip structure | Applications where open rail bolt holes increase contamination or cleaning concerns | Cover-strip configuration, end-retainer space, seals, environment, and block envelope |
| TZ roller guides | High rigidity, static capacity, and moment resistance | CNC machinery, heavy cutting, grinding, injection molding, gantry systems, and high-stiffness automation | Mounting-surface accuracy, preload, moment demand, shock, drive force, and lubrication |
| TG miniature guides | Compact precision motion in standard and wide miniature formats | Medical devices, semiconductor equipment, measuring systems, PCB/IC assembly, and small positioning stages | Small installation envelope, corrosion-resistant material option, sensor space, and mounting screw access |
Practical Starting Points—Not Final Size Rules
For example, a compact optical inspection stage may start with a miniature or low-profile guide because space and moving mass are limited. Similarly, a packaging transfer axis may start with a TO ball guide because speed, smoothness, and standard industrial mounting matter. By contrast, a machining or robot-support axis with high offset moments may need a long TO block, a wider dual-rail layout, or a TZ roller guide.
However, treat these examples only as screening directions. The same industry can contain very different loads and geometries. For instance, a light CNC enclosure door and a cutting-machine table should not follow the same sizing assumption simply because both belong to a CNC machine.
5. Verify Rigidity, Accuracy, Mounting, and Order Data
A guide can pass the load-rating check and still perform poorly when the mounting base, rail alignment, lubrication, protection, or preload does not suit the machine. Therefore, finish size selection with a system review rather than one catalogue lookup.
Preload and Accuracy Do Not Replace Correct Sizing
Preload removes internal clearance and increases rigidity. However, heavier preload also raises rolling resistance and internal stress. Therefore, choose preload from rigidity demand, vibration, accuracy requirements, and mounting quality. Do not use heavy preload to rescue a guide that lacks enough load or moment capacity.
Accuracy grade describes dimensional and running tolerances, but it does not strengthen an inadequate rail profile. Likewise, a larger nominal size cannot guarantee better positioning when the machine base is uneven or the installation team misaligns paired rails. Therefore, review linear guide accuracy and preload guidance after you narrow the mechanical size.
Mounting Quality Can Become the Real Limitation
Moreover, larger and more heavily preloaded guides tolerate less mounting-surface error. For example, a distorted base can force a stiff guide into high running resistance, uneven load sharing, noise, heat, and early wear. In a dual-rail system, the reference rail, driven rail, base flatness, and rail parallelism must form one controlled geometry. Therefore, follow the practical guidance for parallelism and flatness in dual-rail systems.
| Final check | What to confirm before ordering |
|---|---|
| Selected series and nominal size | TO, TT, TF, TZ, or TG; exact size and block family |
| Block type | Square or flange; standard, short, or long; mounting-hole direction |
| Load values | Equivalent load on the most heavily loaded block, not only total payload |
| Moment values | Pitch, yaw, and roll under normal operation, acceleration, and emergency stop |
| Safety factor | Operating condition, shock, vibration, and uncertainty in the external load |
| Rigidity and preload | Required deflection, preload class, drive-force allowance, and mounting quality |
| Accuracy | Running parallelism, pair variation, and equipment-level accuracy target |
| Installation envelope | Rail width, assembly height, block width, block length, and tool access |
| Rail and block layout | Number of rails, number of blocks, rail spacing, and block center distance |
| Environment and protection | Dust, chips, coolant, humidity, temperature, seals, scrapers, cover strip, or coating |
| Physical rail length | Calculate only after the profile, block quantity, and block spacing are fixed |
Information to Send for an Engineering Review
Finally, send a simple machine sketch with the following data. This information reduces back-and-forth and helps the engineering review focus on the most demanding block:
- moving mass and payload;
- external process forces and their directions;
- acceleration, speed, stroke, and duty cycle;
- center-of-gravity offsets from the guide plane;
- rail spacing, block spacing, and mounting orientation;
- maximum available rail width and assembly height;
- required rigidity, accuracy, and expected service life;
- environmental conditions and preferred protection;
- existing model or replacement dimensions, where applicable.
FAQ About Linear Rail Sizes
What do linear rail size numbers mean?
Numbers such as 15, 20, 25, and 35 identify a nominal guide-profile family. Therefore, they help classify compatible rails and blocks, but they do not define every physical dimension or load rating.
Is nominal linear rail size the same as the actual rail width?
Not always. For example, the IMTEK TOH25 family uses a catalogue rail width of 22 mm. Therefore, use the exact dimensional drawing for base machining and replacement checks.
How do I choose between size 15, 20, 25, or 35, 45, 55?
First, calculate the load and moments on the most heavily loaded block. Next, define rail and block spacing. Then compare the required values with catalogue ratings and available installation space. Industry or payload alone cannot select the size.
Can a longer block avoid moving to a larger rail size?
Sometimes. For example, a long block can offer higher load and moment capacity while keeping the same rail profile and assembly height. However, you still need to check the moving envelope, mounting space, load distribution, and rated life.
Does a larger linear rail automatically provide better accuracy?
No. A larger profile can add capacity and rigidity. However, equipment accuracy also depends on accuracy class, preload, base flatness, rail parallelism, drive quality, and assembly procedure.
Should rail length be used to choose the nominal rail size?
No. First, select the nominal profile from load, moment, rigidity, and installation space. Then calculate physical rail length separately from stroke, the complete block-group envelope, clearances, mounting-hole pitch, and end distances.
Can I replace a linear guide with another brand that has the same nominal size?
Do not assume compatibility from the size number alone. Instead, compare rail width and height, assembly height, block mounting dimensions, hole pattern, reference geometry, accuracy class, preload, and rail-to-block compatibility before replacement.
Conclusion
In short, linear rail size is a profile decision rather than a shortcut based on payload or rail width. A sound choice combines the load on each block, pitch/yaw/roll moments, rail and block spacing, installation space, block geometry, static and dynamic ratings, rigidity, preload, and mounting quality.
First, use nominal size to narrow the product family. Next, use the exact catalogue drawing and load tables to confirm the model. Finally, after you fix the rail profile and block layout, continue to the separate linear guide rail length calculation to define physical length and mounting-hole layout.