The main difference between a left hand and right hand ball screw is the direction of the helical groove. This difference changes the relationship between screw rotation and nut travel, so thread direction must be confirmed before a ball screw is manufactured, replaced or installed.
A right hand ball screw and a left hand ball screw can have the same diameter, lead, accuracy grade, material and nut structure, but the nut will move in opposite axial directions under the same defined rotation. Selecting the wrong direction may cause an axis to travel away from its intended position, interfere with another component or fail to match an existing machine.
Descriptions such as "clockwise movement" are not complete unless the viewing end and rotating component are also specified. The following explanations assume that the screw shaft is axially fixed, the ball nut is prevented from rotating and the screw is viewed from the motor or drive end.
Reference condition used below:
The screw rotates, the screw shaft is axially fixed, the nut cannot rotate, and clockwise or counterclockwise rotation is observed from the motor end.
Thread Direction Structure
Thread direction describes the direction in which the helical ball groove rises around the screw shaft. When the shaft is positioned vertically and viewed from the front, a groove that rises toward the right is a right hand thread, while a groove that rises toward the left is a left hand thread.
Left hand and right hand ball screw groove directions viewed from the front
The thread direction does not by itself determine ball screw accuracy, load capacity, preload, axial clearance or service life. These properties depend on the screw size, lead, ball circulation structure, raceway accuracy, material, heat treatment, nut design and operating conditions.
Motion Direction
Under the reference condition stated above, left hand and right hand ball screws produce opposite nut travel from the same screw rotation.
Right Hand Ball Screw
Counterclockwise screw rotation moves the nut toward the motor end. Right hand threads are commonly supplied as the standard direction for general ball screw applications.
Left Hand Ball Screw
Counterclockwise screw rotation moves the nut away from the motor end. A left hand thread is selected when the required motion logic is opposite to that of a right hand screw.
Important: If the observer stands at the opposite end, or if the nut rotates while the screw remains stationary, the apparent relationship will change. Drawings and order documents should therefore identify the viewing end and rotating component.
Identification Methods
1. Observe the Groove Slope
Place the screw shaft vertically and observe it from the front without turning it around. If the visible groove rises from the lower left toward the upper right, it is a right hand thread. If the groove rises from the lower right toward the upper left, it is a left hand thread.
This method is convenient for quick identification, but the shaft position and viewing direction must remain consistent. Turning the screw over or looking from the back can make the slope appear reversed.
2. Perform a Rotation Test
A practical test can be completed before installation:
- Identify and mark the motor or drive end.
- Prevent the ball nut from rotating.
- Keep the screw shaft axially fixed.
- View the screw from the marked motor end.
- Rotate the screw clockwise and record the nut travel direction.
If the nut moves away from the observer under these conditions, the screw is right hand. If the nut moves toward the observer, it is left hand.
3. Check the Model and Product Mark
"LH" is commonly used to identify a left hand ball screw, while "RH" may be used for a right hand version. Because right hand threads are often treated as the standard specification, some models may not include an additional RH mark.
For example, SFU1605-LH normally indicates an SFU series left hand ball screw with a nominal shaft diameter of 16 mm and a lead of 5 mm. The complete interpretation should still be confirmed against the supplier drawing because model coding rules can vary.
DLY left hand and right hand ball screw identification example
On the DLY example shown above, the left hand nut has an additional horizontal identification line on the flange. This marking is used for DLY product identification and should not be assumed to be a universal marking method for every manufacturer.
Video Demonstration
See the Thread Direction in Motion
The video demonstrates how screw rotation is converted into linear ball nut movement. With the viewing end kept unchanged, the movement of left hand and right hand ball screws can be compared under the same rotation.
Pay attention to three conditions during the demonstration: which end is being viewed, whether the screw or nut is rotating, and which component is restrained from rotation.
These details are particularly important when confirming a replacement screw, a mirrored transmission mechanism or a system using opposing linear movement.
Main Differences
| Comparison Item | Right Hand Ball Screw | Left Hand Ball Screw |
|---|---|---|
| Helix direction | Groove rises toward the right when viewed under the defined front-view condition | Groove rises toward the left when viewed under the defined front-view condition |
| Nut travel | Moves away from the motor end during clockwise screw rotation under the stated reference condition | Moves toward the motor end during clockwise screw rotation under the stated reference condition |
| Availability | More commonly used as a standard specification | Often requires separate model, drawing and production confirmation |
| Typical marking | RH or no additional direction mark | LH or a manufacturer-specific identification mark |
| Main reason for selection | Standard axis movement and widely used machine layouts | Opposite, mirrored or layout-specific movement |
| Accuracy and load | Determined by size, accuracy grade, nut design, preload and operating conditions | Determined by size, accuracy grade, nut design, preload and operating conditions |
Application Scenarios
Standard Single-Axis Motion
Right hand ball screws are commonly used in CNC machine feed axes, automation stages, positioning equipment, packaging machinery and linear modules. Their broad availability simplifies replacement, maintenance and standard equipment design.
Centering and Clamping Mechanisms
A left hand and right hand screw arrangement can move two clamping components toward the center or away from the center at the same time. This is useful in centering fixtures, symmetrical grippers, width-adjustment mechanisms and double-sided positioning systems.
Mirrored Machine Layouts
Two machine units may have mirrored mechanical layouts while using motors installed in the same orientation. Selecting opposite screw directions can produce the required mirrored linear travel without changing the overall drive arrangement.
Opposing Synchronous Movement
Left and right hand ball screws can be used in mechanisms where two outputs must travel in opposite directions from a coordinated drive. Depending on the machine structure, this may simplify transmission compared with adding extra reversing gears or separate drive motors.
Space-Limited Equipment
In compact equipment, the motor, coupling, bearing supports and moving table may only fit in one arrangement. A left hand ball screw can provide the required travel direction when changing the motor position or transmission layout is impractical.
Replacement Ball Screws
A replacement ball screw should match the original thread direction unless the control logic and mechanical arrangement are also being changed. Diameter and lead alone are not sufficient for replacement confirmation.
SFU and DFU Selection
Thread direction and nut structure are separate specifications. An SFU ball screw generally uses a single flanged nut and is commonly selected for standard CNC, automation and positioning applications.
A DFU ball screw uses a double-nut flanged structure and may be selected when the application requires greater axial rigidity or improved backlash control.
Choosing SFU or DFU does not automatically determine whether the screw is left hand or right hand. Nut series, thread direction, diameter, lead, accuracy, preload, length and end machining should all be confirmed separately.
Vertical Axis Safety
A left hand ball screw is not inherently more self-locking than a right hand ball screw. Changing the thread direction reverses the motion relationship, but it does not create a reliable load-holding function.
Ball screws use rolling contact and normally have high transmission efficiency. Under an external axial load, a ball screw may be back-driven depending on the lead, efficiency, load, motor condition and transmission arrangement.
Safety requirement for vertical installations
A vertical ball screw axis should use an appropriate brake motor, holding brake, counterbalance, anti-fall mechanism or other independently assessed safety device. Thread direction must not be used as a substitute for load-holding protection.
Ordering Checklist
Providing complete information before quotation and production reduces the risk of receiving a ball screw that fits dimensionally but moves in the wrong direction.
| Confirmation Item | Information to Provide |
|---|---|
| Ball screw series | SFU, DFU or another nut and circulation structure |
| Thread direction | Left hand, right hand or a combined left-and-right arrangement |
| Viewing direction | Clearly mark the motor end or the end from which clockwise rotation is observed |
| Rotating component | Rotating screw or rotating nut |
| Required travel direction | State the nut or table travel direction under the specified motor rotation |
| Nominal diameter | For example, 16 mm, 20 mm, 25 mm, 32 mm or 40 mm |
| Lead | Axial travel per screw revolution, such as 5 mm, 10 mm or 20 mm |
| Required stroke | Actual working travel of the nut or moving table |
| Overall length | Total screw shaft length and effective threaded length |
| Accuracy grade | For example, rolled C7 or ground C5 according to positioning requirements |
| Axial clearance or preload | Standard clearance, controlled clearance or preload requirement |
| End machining | Provide a drawing or complete dimensions for the motor and support ends |
| Support units | BK/BF, FK/FF, EK/EF or another bearing support arrangement |
| Application conditions | Load, speed, mounting direction, duty cycle, operating environment and required service life |
| Quantity | Prototype, replacement quantity or batch order requirement |
Recommended ordering description
Avoid writing only "clockwise rotation moves forward." Instead, state: "Viewed from the motor end, the screw rotates clockwise and the nut must move away from the motor."
Conclusion
Left hand and right hand ball screws differ mainly in the direction of their helical grooves and the resulting relationship between rotation and axial movement. Neither direction is inherently more accurate, rigid or load-resistant.
Right hand ball screws are commonly used for standard linear axes, while left hand ball screws are useful for mirrored layouts, centering mechanisms, opposing movement and machines that require reversed motion under a defined drive direction.
Before ordering, confirm the viewing end, rotating component, required travel direction, nut series, diameter, lead, accuracy, shaft length, end machining and support arrangement. A direction sketch or approved drawing is more reliable than verbal descriptions alone.
Confirm Your Ball Screw Direction
Send the required diameter, lead, length, thread direction, end machining drawing and application conditions for technical confirmation.
WhatsApp: +86 166 0578 8856 | Email: dlyexport2@dlybearing.com
Contact Us
