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Ball Screw vs Lead Screw: Efficiency & Backlash

Every linear motion design starts with the same choice: How do you convert rotary motor motion into linear travel? The two most common answers are the Lead Screw (simple, cheap, friction-based) and the Ball Screw (complex, expensive, rolling-based). Making the wrong choice here is costly. Use a lead screw where you need precision, and you get backlash. Use a ball screw in a vertical lift without a brake, and your load crashes to the floor. In this guide, we compare them side-by-side. Table of Contents 1. The Physics: Sliding vs. Rolling 2. Efficiency & The "Back-Driving" Danger 3. Accuracy and Backlash 4. Selection Table Advertisement 1. The Physics: Sliding vs. Rolling The fundamental difference is friction. Lead Screws rely on Sliding Friction . The nut (often bronze or plastic) slides directly against the steel screw threads. This generates heat and wear. Ball Screws re...
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ISO 286 Limits and Fits: The Complete Engineering Guide

ISO 286 Limits and Fits Diagram
Figure 1: Fundamental deviations for shafts and holes relative to the Zero Line. (Click image to search for the Standard Reference)

In the world of Precision Metrology and CNC machining, adhering to the ISO 286 standard for limits and fits is non-negotiable. Whether you are designing a bearing press fit or a sliding shaft, understanding these metric standards is the difference between a smooth assembly and expensive scrap.

Essential Reference: Most professional engineers rely on the Machinery's Handbook for the complete tables of tolerances and allowances. It is the industry standard for verifying these calculations.
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1. The Big Picture: Hole Basis vs. Shaft Basis

Before calculating numbers, you must choose a system.

  • Hole Basis System (Most Common): We keep the hole size constant (e.g., exactly 20.00 mm with a tolerance of H7) and machine the shaft to fit. This is preferred because drills and reamers come in standard sizes.
  • Shaft Basis System: We keep the shaft size constant (e.g., standard cold-rolled steel bar) and bore the hole to fit. Used for long shafts like textile rollers.

2. Common Fits for Design Engineers

While the standard lists thousands of combinations, 95% of engineering uses just these three:

A. Clearance Fit (Slide)

Code: H7/g6
The shaft is always smaller than the hole. Used for:
Sliding gears, clutch discs, and pivot pins.

B. Transition Fit (Tap)

Code: H7/k6
The zones overlap. It might be tight or loose. Used for:
Locating dowels, pulleys on shafts, and coupling hubs. Requires a light tap with a mallet.

C. Interference Fit (Press)

Code: H7/p6
The shaft is always larger than the hole. Used for:
Permanent bearing mounts, bushings, and seal rings. Requires a hydraulic press or thermal shrink fitting.

3. Decoding the Numbers: IT Grades

The "Number" in the code (e.g., the '7' in H7) tells you the Cost of Manufacturing.

IT Grade Typical Process Application
IT 01-4 Lapping / Superfinishing Gauge blocks, Fuel injectors
IT 5-7 Precision Grinding / Reaming Bearings, Engine pistons
IT 8-11 Turning / Milling General brackets, keyways
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4. Mathematical Calculation (IT Formula)

For general engineering (IT6 - IT16), the standard tolerance unit (i) in microns is calculated as:

i = 0.45 × ∛D + 0.001D

Where D is the geometric mean of the size range.

Tolerance Grade Multipliers

Grade Tolerance Value
IT610i
IT716i
IT825i
IT940i

5. Thermal Considerations (Shrink Fits)

For heavy interference fits (like a railway wheel on an axle), simple pressing is dangerous. We use Shrink Fitting.
By heating the hole (expanding it) and freezing the shaft (shrinking it) using liquid nitrogen, we create a temporary clearance gap.
Formula: ΔL = L × α × ΔT

6. Real-World Calculation Example

Let's calculate the limits for a standard 25 mm diameter shaft with a sliding fit (H7/g6).

Step 1: Identify the Parameters

  • Basic Size: 25.000 mm
  • Hole Tolerance (H7): For 25mm, IT7 = 21 µm (0.021 mm). Since it is 'H', the lower deviation is 0.
  • Shaft Tolerance (g6): For 25mm, IT6 = 13 µm (0.013 mm). The fundamental deviation for 'g' is -7 µm (-0.007 mm).

Step 2: Calculate Limits

Component Max Limit Min Limit Result
Hole (H7) 25.000 + 0.021 25.000 + 0 Ø25.000 - Ø25.021
Shaft (g6) 25.000 - 0.007 (25.000 - 0.007) - 0.013 Ø24.980 - Ø24.993

Step 3: Determine the Fit

Max Clearance: 25.021 - 24.980 = 0.041 mm
Min Clearance: 25.000 - 24.993 = 0.007 mm

Result: The shaft will always slide freely with a small lubricating gap.

Conclusion

A design engineer must balance precision with cost. Specifying an H7 fit on a garden gate hinge is a waste of money; specifying H11 on a turbine bearing is a catastrophe.

📚 Related Engineering Guides

If you found this guide on Limits and Fits useful, check out these related practical design articles:

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