A practical guide for B2B buyers and design engineers on KH support unit bearing housings — covering shaft end fixing methods, alignment accuracy requirements, and mounting configurations for linear bearing slide rail systems. Includes selection criteria, common mistakes, and supplier evaluation questions.
LINEAR MOTION COMPONENTS — BUYER GUIDE
KH Support Unit Bearing Housing: Shaft End Fixing, Alignment Accuracy, and Mounting Configurations for Linear Motion Systems
When you are sourcing a linear bearing slide rail system, most of the attention goes to the rail, the carriage, and the bearing blocks. But the support unit bearing housing — often called the pillow block or end support — is where real-world performance is won or lost. A shaft that is not properly fixed at both ends will flex, vibrate, and wear out the bearing linear slide long before its rated life.
This article covers the three decisions that matter most when selecting a KH support unit bearing housing: how the shaft end is fixed, what alignment accuracy you actually need, and which mounting configuration fits your machine layout. We also include a practical checklist for evaluating suppliers, because in B2B procurement, the drawing is only half the story.
- Shaft end fixing method (fixed vs. floating) determines thermal expansion handling and system rigidity — most applications need one of each.
- Alignment accuracy is a measurable spec (parallelism, concentricity, runout), not a marketing term. Ask for the numbers.
- Mounting configuration (flange, block, pillow, or rail-mounted) affects footprint, load direction, and how easily the linear guide bearing block can be serviced.
- Housing material and seal type matter more in dirty or washdown environments than in clean assembly lines.
- Always request the supplier's tolerance data and test method — a drawing without tolerances is not a specification.
1. Shaft End Fixing: Fixed Side vs. Floating Side
Every linear shaft needs to be constrained at both ends, but the two ends do not do the same job. In a properly designed system, one end is the fixed (or locked) side, and the other is the floating (or supported) side.
The fixed side
The fixed side locks the shaft axially. It carries the thrust load and prevents the shaft from moving along its axis. This is typically achieved with a support unit bearing housing that has a locknut, a retaining ring, or a set screw that clamps the inner ring of the bearing against a shoulder on the shaft.
For KH-type support units, the fixed side usually uses a bearing with a larger contact angle to handle bidirectional axial loads. The housing bore is machined to a tighter tolerance, and the locknut is torqued to a specified value — not just "tight."
The floating side
The floating side allows the shaft to expand and contract with temperature changes. If both ends were fixed, the shaft would bow or push against the bearings, causing premature failure. The floating side uses a bearing that can slide axially inside the housing bore, or a housing with a slightly larger bore to accommodate thermal growth.
For a typical steel shaft, thermal expansion is roughly 12 µm per meter per 10°C. On a 2-meter shaft with a 20°C temperature swing, that is about 48 µm of axial movement. If your floating side cannot absorb that, you will feel it in the form of vibration, noise, and reduced positioning accuracy.
| Parameter | Fixed Side | Floating Side |
|---|---|---|
| Primary function | Axial location, thrust load | Thermal expansion, radial support |
| Bearing type | Angular contact or deep groove with locknut | Deep groove, axial freedom in housing |
| Housing bore tolerance | Tighter (e.g., J7 or K7) | Looser (e.g., H7 or G7) |
| Typical fixing method | Locknut + washer, retaining ring | Slip fit, no axial clamp |
| Common mistake | Using two fixed ends | Using two floating ends |
Buyer tip: When you request a quote, specify whether your application needs a fixed–floating pair or a fixed–fixed arrangement. Many suppliers stock both, but the price and lead time differ. If you are unsure, ask the supplier to recommend based on your shaft length and operating temperature range.
2. Alignment Accuracy: What the Numbers Actually Mean
Alignment accuracy is the single most common source of field failures in linear bearing slide rail systems — and also the most misunderstood spec. It is not one number. It is a set of geometric tolerances that describe how well the housing, the bearing, and the shaft work together.
The three tolerances that matter
Concentricity (coaxiality) — how well the bore centerline of the housing matches the shaft centerline. If these are off, the bearing inner ring will be loaded unevenly, causing localized wear and increased running torque. For a KH support unit, concentricity is typically held to 0.01–0.02 mm depending on the size and grade.
Parallelism — how parallel the mounting surface of the housing is to the shaft axis. If the housing is not parallel to the rail, the linear guide bearing block will bind as it travels. This is especially critical when the support unit is mounted directly to a machined plate without shims.
Runout (TIR — total indicated runout) — the combined error of the bearing, the housing bore, and the shaft journal. This is measured by rotating the shaft and reading a dial indicator on the housing. A typical TIR for a precision KH support unit is 0.02 mm or less.
- Mount the support unit on a surface plate and check the bore height with a height gauge.
- Insert a test shaft and rotate it. Read the dial indicator at the housing bore and at the shaft end.
- Compare the readings against the supplier's stated tolerance. If the supplier cannot provide a test method, treat their numbers as indicative, not guaranteed.
Buyer tip: Ask the supplier for the test method behind their accuracy claims. A reputable manufacturer will describe how they measure runout and parallelism — often with a CMM or a dedicated test fixture. If the answer is vague, that is a red flag.
3. Mounting Configurations: Matching the Housing to Your Machine
The mounting configuration determines how the support unit bearing housing attaches to your machine frame, and it directly affects load capacity, footprint, and serviceability. There is no "best" configuration — only the one that fits your layout and load direction.
Common KH support unit configurations
Pillow block (two-bolt base) — the most common type. The housing has a flat base with two bolt holes, and the shaft passes through the center. It is easy to align, simple to mount, and widely available. Best for horizontal shafts with radial loads.
Flange unit (round or square) — the housing has a flange with bolt holes on the face. It mounts against a vertical plate or a machine wall. Best for vertical shafts or when the shaft must pass through a bulkhead. Flange units are also common on the end of a linear bearing slide rail where space is tight.
Take-up unit (with adjustment) — the housing can be moved along a slot to tension the shaft or belt. Common in conveyor applications where belt stretch is a factor. Not typical for precision linear motion, but worth knowing if you are sourcing for material handling.
Rail-mounted support — the housing integrates with a linear guide bearing block or a rail profile. This is less common for KH-type units and more typical for integrated linear motion systems. If you are building a compact machine, this may save space, but it also limits flexibility.
| Configuration | Typical Load Direction | Best For | Limitations |
|---|---|---|---|
| Pillow block (2-bolt) | Radial (perpendicular to shaft) | Horizontal shafts, general machinery | Requires access to base bolts |
| Flange unit | Radial or axial | Vertical shafts, bulkhead mounting | Larger footprint on the face |
| Take-up unit | Radial + belt tension | Conveyors, tensioning applications | Lower precision |
| Rail-mounted | Multi-directional | Compact integrated systems | Less flexible, harder to source |
Buyer tip: When comparing quotes, do not compare housing prices alone. Compare the complete mounted system — housing, bearing, shaft, and the machining cost of your mounting surface. A cheaper housing that requires a tighter bore tolerance or additional shimming will cost more in the long run.
4. Material and Seal Selection: Environment Drives the Choice
The housing material and seal type are often specified last, but they determine service life in real conditions. For a clean, dry assembly line, a standard cast iron housing with a metal shield is fine. For a washdown environment or a machining center with coolant, you need a different approach.
Cast iron (FC200 or similar) — the default choice. Good damping, low cost, easy to machine. Works well in dry or lightly lubricated environments.
Stainless steel (SUS304 or SUS440C) — for washdown, food processing, or corrosive environments. Higher cost, but necessary where rust would contaminate the product or fail the bearing.
Aluminum alloy — lightweight, used in aerospace or high-speed applications where weight matters. Lower load capacity than cast iron.
Seals — contact seals (e.g., NBR rubber) keep contaminants out but add friction. Non-contact seals (metal shields) have lower friction but let fine dust in. For a linear bearing slide rail in a dusty environment, a contact seal on the support unit and a wiper on the carriage is a common combination.
Buyer tip: Specify the operating environment in your RFQ. If you say "general industrial," the supplier will quote a standard cast iron unit. If you say "food processing with daily washdown," they will quote stainless steel with food-grade grease. The price difference is significant, and the wrong choice is expensive to retrofit.
5. Common Mistakes in Sourcing KH Support Units
We see the same errors repeatedly in procurement and design reviews. Here are the ones that cost the most:
- Using two fixed ends. The shaft has no room to expand, so it bows or pushes against the bearings. Result: noise, vibration, early failure.
- Using two floating ends. The shaft moves axially, losing positioning accuracy. Result: the linear guide bearing block drifts over time.
- Ignoring concentricity between the housing bore and the mounting face. This causes binding and uneven wear on the bearing linear slide.
- Overtorquing the locknut. This distorts the bearing raceway and increases running torque. The locknut should be torqued to the manufacturer's spec, not "as tight as possible."
- Assuming all KH support units are interchangeable. Bore tolerance, housing height, and bolt pattern vary by manufacturer. Always verify the drawing, not just the model number.
6. Supplier Evaluation: Questions to Ask Before You Order
In B2B procurement, the drawing is a promise. The supplier's process is how they keep it. Here is a short checklist to use when evaluating a potential source for KH support unit bearing housings:
| Question | Why It Matters |
|---|---|
| What is the bore tolerance (e.g., J7, K7, H7)? | Determines fit with the bearing outer ring. |
| How do you measure runout and parallelism? | Verifies that accuracy claims are backed by a test method. |
| What is the housing material grade? | Affects load capacity and corrosion resistance. |
| Do you offer both fixed and floating side units? | Ensures you can source a matched pair. |
| What is the lead time for standard and custom sizes? | Affects your production schedule. |
| Can you provide a 2D or 3D drawing for verification? | Confirms bolt pattern, height, and clearance before you machine your mounting plate. |
| What is your MOQ for a mixed order of different sizes? | Affects inventory cost and flexibility. |
7. Final Recommendations
For most linear motion systems, the following starting points will serve you well:
- Use a fixed–floating pair unless your shaft is very short and the temperature is stable.
- Specify a bore tolerance of J7 or K7 for the fixed side and H7 for the floating side.
- Require a runout of 0.02 mm or less for precision applications.
- Choose cast iron for dry environments, stainless steel for washdown.
- Always request the supplier's test method for accuracy claims.
If you are sourcing for a new machine design or replacing an existing linear bearing slide rail system, we recommend sending the supplier your shaft diameter, operating temperature range, load direction, and environment. That information is enough for a competent manufacturer to recommend the correct KH support unit bearing housing configuration — and to quote accurately the first time.
For detailed specifications, dimensional drawings, or to discuss your specific mounting configuration, contact our engineering team. We can provide application guidance and confirm availability for both standard and custom support unit housings.