Before selecting a ball screw model, engineers need to define what the linear axis must actually do. The required load, stroke, speed, positioning accuracy and mounting arrangement determine whether a proposed ball screw will operate safely and achieve the expected motion performance.
A model described only as "20 mm diameter with 5 mm lead" is not a complete specification. Two machines using the same nominal ball screw size may require different accuracy grades, nut structures, support arrangements or end machining because their operating conditions are different.
This guide explains the nine essential ball screw specifications that should be defined before detailed sizing, model selection or quotation.
Start with the Application, Not the Model Number
Ball screw selection normally begins with the machine requirements rather than an existing catalog code. The first step is to describe the motion axis clearly:
- What component will be moved?
- How far must it travel?
- How quickly must the movement be completed?
- What axial load acts on the screw?
- Is the axis horizontal, vertical or inclined?
- What positioning accuracy and repeatability are required?
- How frequently will the machine operate?
These conditions provide the basis for selecting the screw diameter, lead, accuracy grade, nut structure and support method. Choosing a familiar model first and checking the application later can result in insufficient load capacity, excessive shaft speed, buckling risk or unnecessary cost.
1. Axial Load
The ball screw primarily carries load along its axis. Selection therefore requires more than the total weight of the moving component. The manufacturer or designer should evaluate the different forces acting during a complete operating cycle.
Relevant loads may include:
- Weight acting on a vertical or inclined axis
- Acceleration and deceleration force
- Machining or pressing force
- Friction from guides, seals and connected mechanisms
- Impact or temporary peak load
The maximum load helps determine whether the ball screw can withstand peak operating conditions. The average load over the duty cycle is used when evaluating expected fatigue life.
For a vertical axis, the moving mass creates a continuous axial load. Because ball screws can normally be back-driven, the motor brake or an independent holding device must prevent uncontrolled movement during power loss. The ball screw itself should not be treated as a self-locking safety mechanism.
2. Required Stroke and Overall Length
Required stroke is the distance the nut must travel. It is not the same as the total ball screw length.
The complete shaft normally includes:
- Usable nut travel
- Additional threaded length outside the working stroke
- Fixed-end bearing seat
- Supported-end bearing seat
- Motor or coupling connection
- Shoulders, threads, grooves or other machined features
Longer shafts require additional attention because unsupported length influences both critical speed and resistance to buckling. When requesting a quotation, state the required stroke and total length separately rather than providing only one length value.
3. Linear Speed and Acceleration
The required linear speed affects the ball screw lead and rotational speed. Their basic relationship is:
Rotational speed = Linear speed ÷ Ball screw lead
When units are converted consistently, a screw moving at 500 mm/s with a 10 mm lead must rotate at 3,000 rpm. If the same motion uses a 5 mm lead, the required rotational speed increases to 6,000 rpm.
A smaller lead does not automatically provide a better system. It may improve theoretical mechanical resolution and increase available linear thrust for a given motor torque, but it also requires higher shaft speed to achieve the same linear velocity.
Acceleration should also be defined. Rapid acceleration increases the torque required from the motor and creates additional force in the ball screw, support bearings and connected structure.
4. Screw Diameter
The nominal screw diameter affects axial load capacity, shaft rigidity, critical speed and buckling resistance. A larger diameter generally provides greater rigidity and is better suited to longer strokes or higher compressive loads, but it also increases inertia, installation space and cost.
Diameter selection should therefore consider:
- Maximum and average axial load
- Unsupported shaft length
- Maximum rotational speed
- Horizontal or vertical installation
- End-support arrangement
- Available mounting space
For long vertical or compressively loaded screws, buckling verification is particularly important. For long high-speed shafts, critical-speed verification may become the main limitation instead.
5. Ball Screw Lead
The lead is the linear distance traveled by the nut during one complete revolution of the screw. It directly connects motor rotation with linear movement.
| Lead Choice | General Effect | Points to Check |
|---|---|---|
| Smaller lead | More linear force per unit of input torque and smaller travel per motor revolution | Higher screw rpm is required for the same linear speed |
| Larger lead | More linear travel per revolution and lower screw rpm for the same linear speed | More motor torque may be required for the same axial thrust |
The controller resolution, motor speed, required thrust and target cycle time should be considered together. Lead selection should not be based on positioning accuracy alone.
6. Rolled or Ground Ball Screw
The required positioning performance and project cost help determine whether a rolled or ground ball screw is appropriate.
Comparison of ground and rolled ball screw manufacturing methods
A rolled ball screw has its shaft raceway formed through plastic deformation using rolling dies. It provides efficient production and an economical solution for general industrial motion. DLY commonly supplies cold-rolled ball screws in C7 accuracy for automation equipment, packaging machines, CNC routers and other standard applications.
A ground ball screw has its raceway finished through precision grinding. It is generally selected when the machine requires tighter lead control, more demanding positioning performance or higher motion consistency.
The manufacturing method should be chosen according to the required axis accuracy. Using a ground screw cannot compensate for errors caused by an unsuitable support structure, poor alignment, flexible mounting surfaces or inaccurate machine assembly.
7. Accuracy, Repeatability and Axial Clearance
These terms describe different aspects of motion performance and should not be used interchangeably.
| Term | Meaning | Machine Effect |
|---|---|---|
| Lead accuracy | Difference between actual and theoretical travel over a defined length | Influences absolute positioning over the stroke |
| Repeatability | Ability to return repeatedly to the same commanded position | Important for repeated machine cycles |
| Axial clearance | Free axial movement between the screw and nut under reversing force | Influences direction-reversal accuracy |
| Preload | Internal load applied to reduce clearance and increase rigidity | Improves reversal response but increases friction and driving torque |
Preload terminology and available levels vary by manufacturer and nut design. A buyer should specify the required axial clearance, rigidity or running-torque condition rather than assuming that one preload code has the same meaning across all suppliers.
8. Mounting Orientation and Support Method
The same ball screw may have very different operating limits depending on how its ends are supported. Common arrangements include fixed–free, supported–supported, fixed–supported and fixed–fixed configurations.
A fixed-end support normally contains a preloaded angular-contact bearing arrangement that locates the shaft axially. The opposite support accommodates the non-driven end and helps control shaft deflection. Suitable support units may include BK/BF, FK/FF or EK/EF combinations, depending on the screw size and mounting structure.
The support method affects:
- Permissible rotational speed
- Buckling resistance
- Axial rigidity
- Thermal expansion behavior
- Installation complexity
The orientation must also be stated. A vertical axis requires consideration of gravitational load, motor braking and anti-fall protection. A horizontal long-stroke installation may require additional control of shaft sag and critical speed.
9. End Machining and Connected Components
Standard ball screw shafts often require machining before they can be installed. Both shaft ends must match the selected bearing supports, locknut, motor coupling and surrounding machine structure.
Typical ball screw shaft end machining features
An end-machining drawing should define:
- Bearing-seat diameters, lengths and tolerances
- Shoulder positions
- External or internal threads
- Locknut and retaining-ring grooves
- Coupling diameter and connection length
- Keyways, flats or wrench sections
- Required center holes and chamfers
Do not select the ball screw independently from its support bearings and coupling. A dimensionally correct screw shaft can still produce poor motion if the bearing arrangement lacks rigidity or the end machining creates misalignment.
DLY can supply ball screws with customized end machining and compatible ball screw support units according to the customer's assembly drawing.
Additional Operating Conditions
The nine main specifications define the mechanical requirements, but the surrounding environment must also be considered before final model confirmation.
Important conditions include:
- Operating temperature
- Dust, chips, coolant or moisture exposure
- Lubricant type and maintenance interval
- Continuous or intermittent operation
- Required service life
- Noise limitations
- Cleanroom or corrosion-resistance requirements
Lubrication intervals cannot be defined by one universal operating time. Relubrication depends on speed, stroke, load, temperature, contamination and lubricant condition. Equipment manufacturers should establish an interval based on the actual duty cycle and inspect the lubrication condition during commissioning.
How the Specifications Work Together
Ball screw parameters should not be selected independently. Each choice changes another part of the system:
- A longer stroke may require a larger shaft diameter or a different support method.
- A higher linear speed may require a larger lead to keep shaft rpm below the critical-speed limit.
- A smaller lead may increase available thrust but also increase the rotational speed needed for the same travel rate.
- Higher preload may improve rigidity but increase friction, temperature and motor torque.
- A higher accuracy grade may improve lead control but cannot eliminate installation and structural errors.
The final selection should therefore be checked as a complete drive system that includes the ball screw, nut, support bearings, coupling, motor, guides and machine structure.
Conclusion
Before choosing a ball screw, define the axial load, stroke, speed, acceleration, screw diameter, lead, accuracy requirement, axial clearance, mounting arrangement and end machining. These specifications provide the necessary input for checking service life, buckling, critical speed and motor requirements.
For a replacement project, send the complete model number, dimensional drawing and installation information. For a new machine, provide the working conditions rather than selecting a catalog model from diameter and lead alone.
DLY supplies C7 rolled and C5 ground ball screws, standard and customized ball nuts, end machining and matching support components. Our engineering team can review your application data and recommend a suitable ball screw configuration.
