How to Select Linear Rail Length

Oct 9

The correct linear rail length is determined by more than required machine stroke. You must account for carriage length, carriage spacing, end-of-travel position, mounting-hole pitch, end-hole distance, structural support, rail manufacturing limits, and any allowance needed for overtravel or future adjustment.

A useful first approximation is:

Rail length ≈ required carriage travel + moving carriage footprint + end allowance

But that is only the starting point.

The final rail length should also match a practical mounting-hole pattern and stay within the manufacturer’s allowable rail-length range. Schaeffler’s current linear-guide calculator similarly works from the complete required stroke and returns the shortest rail length that can accommodate it, with additional buffer added where desired.

Key Takeaways

Rail length is not the same thing as stroke.
The carriage or carriage group must remain fully supported throughout travel.
The moving carriage footprint must be added to required stroke.
Multiple blocks require their complete outside-to-outside footprint to be considered.
Add end allowance where needed for safety, seals, stops, or adjustment.
Rail mounting-hole pitch can determine the most practical final cut length.
End-hole spacing should not be improvised without checking the manufacturer’s dimensional system.
Very short end sections can reduce mounting stability.
Manufacturer maximum rail length varies by guide family and accuracy grade.
Long rails may require jointed sections.
Two parallel rails should normally be supplied or cut to matching functional lengths.
Longer-than-required rail is not automatically better.
For cut-to-length replacement work, preserve the original mounting-hole geometry whenever possible.

Why Stroke Alone Does Not Determine Rail Length

Suppose a machine requires:

500 mm of travel.

It would be easy to assume:

500 mm stroke = 500 mm rail.

That is usually wrong.

The carriage itself occupies part of the rail.

If the carriage is:

100 mm long,

and it must move:

500 mm,

the rail must be long enough for that carriage to remain supported at both ends of its travel.

A simplified minimum becomes:

500 mm travel + 100 mm carriage length = 600 mm rail

before considering:

safety margin,
mounting-hole pattern,
end spacing,
stops.

The Basic Rail-Length Formula

For one carriage:

Lrail ≈ S + B + 2E

where:

Lrail = required rail length,
S = required stroke,
B = carriage length,
E = desired end allowance on each side.

Example

Required stroke:

600 mm

Carriage length:

90 mm

End allowance:

20 mm per side

Then:

Lrail = 600 + 90 + 40

Lrail = 730 mm

You would then adjust that theoretical value to a manufacturable rail length with an appropriate mounting-hole pattern.

Multiple Blocks Change the Calculation

If the moving table uses two blocks on each rail, use the total carriage-group footprint, not the length of one block.

Suppose:

each block = 80 mm long,
center-to-center block spacing = 250 mm.

The outside-to-outside footprint is approximately:

250 + 80 = 330 mm

If required stroke is:

700 mm

then the theoretical minimum rail length before end allowance becomes:

700 + 330 = 1,030 mm

Carriage Group Footprint Formula

For two identical blocks:

Bgroup = D + B

where:

D = center-to-center block spacing,
B = block length.

Then:

Lrail ≈ S + Bgroup + 2E

This is one of the most useful practical formulas for a two-block rail.

Why Block Spacing Matters

Block spacing is normally chosen for:

moment resistance,
rigidity,
load distribution.

But it also increases required rail length.

If you increase block spacing from:

200 mm

to:

400 mm,

the carriage group's footprint becomes roughly:

200 mm longer.

The rail usually needs to grow by approximately the same amount if stroke is unchanged.

This creates a machine-design tradeoff:

greater block spacing improves moment leverage but increases rail and machine length.

Example: Two Blocks on Each Rail

Assume:

required stroke = 800 mm,
block length = 105 mm,
block center spacing = 300 mm,
end allowance = 25 mm each side.

Block-group footprint:

300 + 105 = 405 mm

Rail length:

800 + 405 + 50

= 1,255 mm

That is the geometric requirement.

The next step is to determine whether:

1,255 mm

is practical with the selected rail's mounting-hole pitch.

Why Mounting-Hole Pitch Matters

Profile rails typically have mounting holes arranged at a regular pitch.

A rail may use a pitch such as:

60 mm,
80 mm,
another manufacturer-specific value.

The exact value depends on:

brand,
series,
size.

For example, THK publishes standard rail lengths alongside a standard mounting pitch and end dimension for each model. On one current HSR-M1 example, THK lists 60 mm pitch for sizes 15–25 and 80 mm pitch for sizes 30–35.

That does not mean those pitches apply to all guides.

Always use the exact model's dimensional table.

Rail Length and End-Hole Spacing

A typical cut rail has:

total length L,
mounting-hole pitch P,
end dimension G or equivalent.

Conceptually:

L = G1 + nP + G2

where:

G1 = distance from first rail end to first mounting-hole center,
G2 = distance from final mounting-hole center to opposite rail end,
P = mounting-hole pitch,
n = number of spaces between holes.

For a symmetrical cut:

G1 = G2

but that is not mandatory if the machine requires a different end-hole layout.

Why Very Large End Distances Can Be a Problem

Rail ends need adequate structural support.

THK specifically notes in its current rail-length guidance that increasing the end dimension can make the unsupported portion less stable and can adversely affect accuracy.

That means a rail should not simply be:

cut anywhere between holes

without considering where the first and last bolts support it.

Rail Cut-Length Example

Suppose the selected rail has:

P = 60 mm mounting-hole pitch

and you would like:

G1 = G2 = 20 mm.

A practical length might follow:

L = 20 + n(60) + 20

If n = 20:

L = 20 + 1,200 + 20

L = 1,240 mm

This is why catalog standard lengths often appear in seemingly unusual numbers such as:

1,180 mm,
1,240 mm,
1,300 mm.

They reflect the mounting-hole pattern.

Why the Nearest Catalog Length May Be Better

Suppose your geometric calculation says:

1,217 mm.

If the manufacturer offers a standard:

1,240 mm

rail with a well-balanced mounting-hole layout, the 1,240 mm rail may be the better choice.

Advantages can include:

proper end-hole distances,
easier ordering,
standard manufacturing,
better support,
less custom processing.

Do not trim a rail merely to remove 23 mm unless the machine envelope requires it.

When Custom-Cut Rail Makes Sense

Custom rail length is useful when:

machine envelope is fixed,
replacing an obsolete rail,
matching existing holes,
maximizing stroke in limited space,
machine rebuild requires exact dimensions.

Schaeffler's current Linear Guide Calculator notes that customized rail lengths can be specified in 1 mm increments for applicable systems and distinguishes standard, customized, maximum, and jointed rail arrangements.

Linear Automation USA also provides a Linear Rail Cut Calculator for planning cut rails around variables such as:

rail size,
mounting-hole pitch,
buffer/end dimensions,
total length.

Stroke vs. Usable Stroke

It is helpful to distinguish:

rail length

from:

usable carriage stroke.

A rail can be 1,000 mm long without providing 1,000 mm carriage travel.

If the moving carriage group occupies:

300 mm,

the absolute geometric travel before overhang would be approximately:

700 mm

before adding end margins.

Maximum Geometric Stroke

Conceptually:

Smax ≈ Lrail − Bgroup − 2E

Example:

Rail:

1,200 mm

Block-group footprint:

350 mm

End margin:

25 mm each side

Then:

Smax ≈ 1,200 − 350 − 50

Smax ≈ 800 mm

Why You Should Include End Allowance

A rail designed with zero geometric margin leaves no allowance for:

travel switches,
mechanical stops,
deceleration zone,
assembly variation,
protective seals,
future adjustment.

Even if the carriage can technically reach the extreme end, that may not be desirable.

How Much End Allowance Should You Add?

There is no universal value.

It depends on:

machine layout,
guide type,
control system,
physical stops,
end seals,
required adjustment.

Do not invent an arbitrary 25 mm or 50 mm rule and apply it universally.

Define the required usable stroke first, then decide how much physical reserve the machine needs.

Mechanical Stops Matter

A servo-controlled machine should not depend solely on the rail length to prevent a carriage from leaving the rail.

Consider:

programmed travel limits,
limit switches,
hard stops,
collision conditions.

The rail should accommodate the intended movement envelope without allowing normal operation to approach an unsafe physical condition.

Do Not Let a Carriage Run Off the Rail

Profile rail blocks contain recirculating rolling elements.

Removing or overrunning the block can cause:

balls or rollers to be displaced,
seal damage,
contamination entry.

THK specifically provides precautions for preventing blocks from falling off rails on many current guide families.

Do not design normal machine stroke around deliberate carriage overrun unless the guide is specifically designed for that operating mode.

Standard Rail Length vs. Maximum Rail Length

Manufacturers typically publish:

standard rail lengths,
maximum single-piece rail length.

These values vary by:

series,
size,
accuracy.

THK, for example, states that maximum manufactured rail length can be limited when a high degree of precision is required.

This matters on:

long machine tools,
gantries,
transfer lines,
large automation systems.

Example of Manufacturer Length Limits

THK's current HSR-M1 data shows model-dependent maximum single-rail lengths around:

1,240 mm,
1,480 mm,
1,500 mm

for the special high-temperature models shown in its table, and states that longer requirements may use jointed rails.

These numbers are specific to that family.

Other profile guide families may allow substantially different rail lengths.

Do not generalize those limits.

What Are Jointed Linear Rails?

When the required travel exceeds a practical single-piece rail length, manufacturers can use:

jointed rails.

Multiple rail sections are manufactured or prepared so they align as one continuous guideway.

This is common for:

long-travel gantries,
machine tools,
automation transfer systems.

Jointed Rails Require Precision

Joining rails is not the same as placing two ordinary cut rails end-to-end.

Critical concerns include:

alignment,
rail-end geometry,
raceway transition,
mounting accuracy,
section sequence.

Schaeffler's current calculator specifically treats joint rails as a customized rail-length solution rather than simply another standard rail choice.

Should You Buy One Very Long Rail or Two Jointed Rails?

Use a single rail where practical.

A single-piece rail avoids:

joint alignment,
transition geometry,
section matching.

But jointed rails may be required because of:

manufacturing limits,
shipping limits,
installation access,
machine length.

Use manufacturer-supported jointing procedures.

Rail Length and Accuracy

Longer rail systems can be more demanding to install accurately.

As rail length increases, controlling:

straightness,
mounting-base flatness,
parallelism between rails

becomes increasingly important.

High-accuracy guide systems may also have lower maximum manufacturable lengths, as THK notes in its current ordering documentation.

Rail Length Does Not Determine Accuracy Class

A longer rail is not inherently:

less accurate,
more accurate.

Accuracy class remains a separate specification.

But the machine base must maintain the required geometry across the complete rail length.

Two Parallel Rails Should Be Planned Together

If your system uses two rails:

Rail A,
Rail B,

their functional dimensions should usually be coordinated.

Verify:

total length,
hole pitch,
end distances,
mounting datum.

For new machinery, matching lengths simplifies:

mounting,
alignment,
table geometry.

Do Both Rails Have to Be Exactly the Same Length?

Not always.

A machine design can intentionally use different rail lengths.

But unless the design requires it, matching rails are normally simpler and easier to service.

For replacement work, duplicate the original functional layout whenever possible.

Rail Length and Moving-Rail Systems

Not every machine has stationary rails.

Sometimes:

carriage is fixed,
rail moves.

The same geometric principle still applies:

The rail must remain adequately engaged with the block throughout the full required travel.

But the moving mass changes, which can affect:

acceleration forces,
actuator sizing,
system dynamics.

THK includes moving-rail and various mounting arrangements among the conditions that should be established during guide selection.

Rail Length and Carriage Length

Longer carriages require longer rails for the same stroke.

Suppose:

Standard Block

Block length:

90 mm

Long Block

Block length:

130 mm

Required stroke:

600 mm

With one carriage and identical margins:

The long-block version requires approximately:

40 mm more rail length.

That machine-envelope consequence should be considered when choosing standard vs. long blocks.

Rail Length and Number of Blocks

Adding more blocks can also increase rail length.

Suppose:

original axis has one 100 mm carriage,
upgraded axis has two 100 mm carriages spaced 250 mm apart.

Original moving footprint:

100 mm

New footprint:

350 mm

For identical stroke, rail requirement increases by roughly:

250 mm.

Rail Length and Moment Capacity

Why use the extra length?

Because increased block spacing can improve:

pitch rigidity,
yaw rigidity,
load distribution.

So a longer rail can indirectly improve machine performance by allowing a larger carriage support footprint.

But the benefit comes from:

block spacing

not simply from the rail being longer.

Don't Add Rail Length Without Purpose

Excess rail length can increase:

material cost,
shipping difficulty,
machine footprint,
mounting preparation,
exposure to contamination.

If an axis only needs:

500 mm travel

there is normally no benefit to installing a:

2,000 mm rail

unless machine geometry or future expansion requires it.

Future Stroke Expansion

Sometimes a designer intentionally chooses extra rail length to allow:

later table repositioning,
additional tooling,
future stroke increase.

That can be reasonable.

If so, design it deliberately.

Do not accidentally create long unsupported rail ends or poor mounting-hole positions.

Rail Length for Replacement Applications

Replacement is different from new design.

For an existing machine, try to document:

original rail length,
rail width,
rail height,
hole pitch,
first-hole distance,
last-hole distance,
hole diameter,
counterbore dimensions.

The machine base is already drilled.

Matching the original mounting geometry can dramatically reduce installation work.

Never Order a Replacement Rail From Length Alone

A statement such as:

"I need a 1,200 mm size-25 rail"

is incomplete.

You still need:

manufacturer,
series,
rail profile,
hole pitch,
end-hole geometry.

Nominal size and rail length do not establish interchangeability.

Cut-to-Length Rail Example

Suppose an existing machine uses:

size 25 profile rail,
length = 1,340 mm,
pitch = 60 mm,
first-hole distance = 40 mm,
last-hole distance = 40 mm.

Before cutting a replacement, verify that:

40 + n(60) + 40 = 1,340

Here:

1,340 − 80 = 1,260

1,260 / 60 = 21

So the mounting pattern contains:

21 pitch intervals

between the first and last hole centers.

This is a complete, reproducible rail-cut specification.

What If the Existing Rail Has Unequal End Dimensions?

Suppose:

total length = 1,315 mm,
pitch = 60 mm,
G1 = 35 mm.

If there are 21 intervals:

G2 = 1,315 − 35 − (21 × 60)

G2 = 20 mm

That may have been intentional to line up with:

machine frame,
end stops,
existing mounting holes.

Do not automatically recenter the hole pattern.

Linear Rail Cut Calculator Logic

A useful cut calculation is based on:

total rail length L,
hole pitch P,
end distance G1,
corresponding opposite end G2.

Linear Automation USA's Resources page includes its Linear Rail Cut Calculator for planning these relationships.

For custom industrial replacements, this is often more useful than specifying rail length alone.

How to Select Rail Length for a New Machine

Use this sequence.

Step 1: Define Required Usable Stroke

Start with the actual machine travel.

Not nominal actuator stroke.

Not desired rail length.

Required usable motion.

Step 2: Select Carriage Configuration

Determine:

one block,
two blocks,
four-block arrangement.

Step 3: Determine Carriage Length

Use the exact manufacturer dimension.

Step 4: Determine Block Spacing

Set spacing based on:

load,
pitch/yaw moments,
rigidity.

Step 5: Calculate Moving Footprint

For two blocks:

Bgroup = block spacing + block length

Step 6: Add Required Stroke

Lminimum = stroke + moving footprint

Step 7: Add End Allowance

Include necessary:

overtravel margin,
stops,
machine adjustment.

Step 8: Check Mounting-Hole Pitch

Adjust length so the first and last mounting holes provide a suitable rail-support pattern.

Step 9: Check Machine Envelope

Make sure the resulting rail fits:

base,
guards,
cable carrier,
actuator.

Step 10: Check Maximum Manufacturer Length

Confirm the required length can be made as:

one piece,
jointed rail.

Step 11: Verify Accuracy Requirements

Long, high-precision rails may require special manufacturing considerations.

Step 12: Finalize G1 and G2

Document the end-hole dimensions.

Worked Example: New Industrial Axis

Machine requirements:

usable stroke = 1,000 mm,
two blocks per rail,
block length = 110 mm,
block spacing = 350 mm,
desired end reserve = 25 mm each side.

Calculate Group Footprint

350 + 110 = 460 mm

Add Stroke

1,000 + 460 = 1,460 mm

Add End Reserve

1,460 + 50 = 1,510 mm

Initial target:

1,510 mm

Suppose the selected rail has:

80 mm mounting pitch.

Instead of arbitrarily ordering exactly 1,510 mm, use the manufacturer's dimensional data to determine a practical:

total length,
hole count,
G1,
G2.

A nearby manufacturable value may provide a much better mounting pattern.

Worked Example: Tight Machine Envelope

Available rail space:

900 mm

Required stroke:

600 mm

This leaves:

300 mm

for:

carriage group,
margins.

If the planned two-block group occupies:

350 mm,

the design cannot physically provide 600 mm of fully supported travel.

Something must change:

shorten block spacing,
choose shorter carriages,
reduce stroke,
increase machine envelope.

This is why rail length should be considered early in machine design.

Worked Example: Long-Block Upgrade

Current:

rail = 1,200 mm,
stroke = 800 mm,
block group = 350 mm,
margins = 50 mm total.

Required:

800 + 350 + 50 = 1,200 mm

Now replace blocks with a long version that increases group footprint by:

60 mm.

New requirement:

1,260 mm

The old rail may no longer provide the original usable stroke.

Changing block type can therefore change rail-length requirements.

Common Linear Rail Length Mistakes

Mistake 1: Making Rail Length Equal to Stroke

The carriage itself occupies rail length.

Mistake 2: Using One Block Length When There Are Two Blocks

Use the full carriage-group footprint.

Mistake 3: Ignoring Mounting-Hole Pitch

Theoretical length may produce poor end-hole geometry.

Mistake 4: Cutting Directly Through the Existing Pitch Pattern Without Planning

Determine G1 and G2 first.

Mistake 5: Leaving Excessive Unsupported Rail Ends

Rail end support can affect stability and accuracy.

Mistake 6: Ordering a Rail Longer Than the Manufacturer's Single-Piece Limit

Verify maximum rail length.

Mistake 7: Improvising Jointed Rails

Use manufacturer-supported jointed sections.

Mistake 8: Ignoring Long-Block Length

Longer carriages consume more rail.

Mistake 9: Forgetting Future Mechanical Stops

Usable travel should not end at physical rail departure.

Mistake 10: Ordering Replacement Rail From Size and Length Alone

Part family and mounting geometry also matter.

Linear Automation USA's Perspective

At Linear Automation USA, we treat rail length as a machine-geometry and mounting-pattern decision, not simply a saw-cut dimension.

We start with:

required stroke → carriage footprint → end allowance → mounting pitch → G1/G2 → final rail length

That sequence prevents a common mistake:

calculating the perfect mechanical stroke and then discovering that the requested rail length leaves an awkward or poorly supported mounting-hole pattern.

For replacement applications, we go one step further.

Our preference is to preserve the machine's original:

rail length,
hole pitch,
first-hole position,
last-hole position

when practical.

That can allow a replacement rail to install into the existing machine base without:

drilling new holes,
relocating stops,
modifying the table.

We also distinguish between:

standard rail length

and:

custom cut length.

A standard length can be the best choice when it fits the machine.

A custom-cut rail is valuable when the machine requires:

an exact envelope,
exact existing hole locations,
maximum usable travel.

Linear Automation USA supports profile rail systems from Schaeffler, SBC Linear, and WON Linear, along with local cut-to-length rail support.

Our rule is:

Select rail length from usable stroke and carriage geometry first, then finalize the cut from the rail's mounting-hole pattern—not the other way around.

Frequently Asked Questions

How Long Should a Linear Rail Be?

Long enough to accommodate the required stroke, full moving carriage footprint, necessary end margins, and a suitable mounting-hole layout.

Is Rail Length the Same as Stroke?

No. Rail length must normally be longer than usable stroke because the carriage occupies part of the rail.

How Do I Calculate Minimum Linear Rail Length?

A useful starting point is:

rail length = stroke + carriage-group footprint + end allowances.

Then adjust for mounting-hole pitch and manufacturer requirements.

How Much Longer Should the Rail Be Than the Stroke?

That depends on carriage length, number of blocks, block spacing, and end margins. There is no universal fixed amount.

Do Two Linear Guide Blocks Require a Longer Rail?

Usually yes. Their combined outside-to-outside footprint is greater than one block.

What Is G1 on a Linear Rail?

G1 commonly refers to the end distance from the rail end to the first mounting-hole center, though terminology varies by manufacturer.

Should G1 and G2 Be Equal?

Not necessarily. Equal ends are convenient, but asymmetric end distances may be required for a particular machine.

Can Linear Rails Be Cut to Any Length?

Many profile rails can be cut to custom lengths, but the final cut should respect mounting-hole pitch, suitable end distances, manufacturer limits, and machining practices.

Can I Cut Through a Mounting Hole?

That is generally undesirable. Final rail length should be planned around a workable hole pattern.

Can Linear Rails Be Joined?

Yes, manufacturer-supported jointed rails are available for many long-travel systems.

Does Rail Length Affect Accuracy?

Length itself is not an accuracy class, but long rails and high-precision systems impose greater demands on manufacturing and installation. Some manufacturers apply accuracy-related maximum rail-length limits.

Does a Long Block Reduce Available Stroke?

Yes. If rail length remains unchanged, a longer block or wider block spacing reduces available carriage travel.

Can I Order Two Different-Length Rails for One Axis?

Yes if the machine is designed that way, although matching functional rail lengths are normally simpler in conventional two-rail systems.

Need Help Selecting or Cutting Linear Rail Length?

For a new application, record:

required usable stroke,
rail manufacturer,
rail series,
rail size,
carriage model,
carriage length,
number of blocks,
block spacing,
number of rails,
machine envelope,
desired end reserve.

For replacement rails, also record:

exact original rail length,
mounting-hole pitch,
G1,
G2,
hole diameter,
counterbore dimensions,
clear photographs.

Contact Linear Automation USA with those dimensions when you need help specifying a replacement or custom-cut profile rail.