Husillo de bolas frente a piñón y cremallera: cómo elegir para máquinas CNC

Revisión técnica: Mayla Wang, jefa del departamento técnico, Jinan Longqiao Electromechanical Equipment Co., Ltd.

Ball screw and rack and pinion: what they are

Ball screw. A ball screw turns rotation into linear motion through a screw shaft and a nut. Recirculating steel balls roll between the two threads instead of sliding, so friction is low and efficiency is high, typically around 90 percent. With a preloaded nut the clearance between nut and shaft is removed, which gives very low backlash and high axial stiffness. That is why ball screws are the standard drive for the Z axis and for the X and Y axes of small and medium CNC machines.

Rack and pinion. A pinion gear mounted on the motor or gear reducer meshes with a toothed rack fixed to the machine. The pinion travels along the rack, so the stroke is limited only by how many rack sections you bolt end to end. Racks are the usual choice for the long axis of gantry routers, plasma and laser machines and wood or stone CNC machines, where a ball screw would be too long to run fast without whipping.

Lead screw (for reference). A lead screw uses sliding contact between a threaded rod and a nut. It is cheap and quiet but has higher friction, lower efficiency and faster wear, so it suits light loads and low duty cycles rather than production CNC axes. See our article on ball screw vs lead screw for details.

Ball screw vs rack and pinion vs lead screw

A simple rule: if the axis is shorter than about 1.5 to 2 m and accuracy is the priority, use a ball screw. If it is longer than about 2 to 3 m, or the machine needs fast rapids over a large bed, use a rack and pinion. Between those values, decide by required accuracy, speed and budget. Many machines use both: a ball screw on Z and a rack on the long X axis.

How to choose a ball screw: 6 steps

1. Define stroke and travel speed. Note the longest travel, the rapid speed you want in m/min and the motor speed you can run.
2. Pick the lead. Lead is how far the nut moves per screw revolution. Linear speed equals lead times rpm. A 5 mm lead gives finer resolution and more thrust from the same motor; a 10 mm lead gives double the speed at the same rpm but needs more torque and has coarser resolution.
3. Choose the diameter. Larger diameter gives higher stiffness, higher load rating and higher critical speed. Common CNC sizes are 16, 20, 25 and 32 mm.
4. Check critical speed. The screw must run well below the speed at which it starts to whip. This depends on length, root diameter and end supports (see next section).
5. Select the end supports. Fixed-fixed is stiffest and gives the highest critical speed. Fixed-simple is the usual compromise. Fixed-free is only for short, light screws.
6. Choose accuracy grade and preload. C5 for precision axes, C7 for general CNC and automation. Use a preloaded nut when backlash must be kept very small.

Critical speed, with a worked example

Critical speed is the rotation speed at which the screw begins to vibrate heavily, like a skipping rope. A common estimate is:

N_c = f × 1.2 × 10⁸ × d / L²

where N_c is in rpm, d is the screw root diameter in mm, L is the unsupported length between supports in mm, and f depends on the end supports: 0.36 for fixed-free, 1.0 for simple-simple, 1.47 for fixed-simple and 2.23 for fixed-fixed. Run the screw at no more than about 80 percent of this value.

This is a rough engineering estimate for explaining the method, not a catalogue rating. If you need higher speed on a long axis, increase the diameter, shorten the unsupported length, use fixed-fixed supports or switch to a rack and pinion. Send us your stroke and speed and our engineers will check the numbers for your machine.

SFU ball screw sizes and accuracy grades

SFU is the common single-nut ball screw series used on CNC routers, engraving machines and small machining centres. Typical designations read as diameter followed by lead, so SFU1605 is a 16 mm screw with a 5 mm lead.

Our Husillo de bolas SFU is offered in C5 and C7 accuracy grades, in carbon steel with a milled thread and a bearing-steel nut, with a preload design available. Standard lengths and the full range of sizes are listed on the product page, and custom lengths and end machining can be made from your drawing. A machinery test report can be provided. See also our SFU ball screw size chart.

How to choose a rack and pinion

1. Pick the module. Module (M) sets tooth size; pitch equals module times π. Larger modules carry more load. Common CNC modules include 1, 1.25, 1.5, 2 and 3. Our standard cremallera y piñón uses module 1.25.
2. Choose straight or helical teeth. Helical teeth engage gradually, so they run quieter and smoother and handle higher speed. Straight teeth are cheaper and easier to align.
3. Check the material and hardening. Our racks use C45 steel, with S45C and SCM440 also available, and high-frequency hardened teeth for wear resistance.
4. Calculate speed. Linear speed = π × module × pinion teeth × pinion rpm. For module 1.25 and a 24-tooth pinion the pitch diameter is 30 mm, so one pinion revolution moves the axis about 94 mm.
5. Add a gear reducer. A planetary reducer multiplies torque, lowers reflected inertia and gives finer positioning. See our reductor planetario range and the article on choosing a servo and reducer.
6. Plan the joints. Racks are supplied in sections. Use a pitch-setting jig when joining so the tooth spacing stays continuous across the joint.

A worked illustration: with a 30 mm pitch diameter pinion, a 5:1 reducer and a motor at 3,000 rpm, the pinion turns 600 rpm and the axis moves at about 56 m/min. Peak speed in real machines is limited by acceleration, machine mass and the controller, so treat this as a ceiling, not a target.

Reducing rack backlash

Because gear teeth need a little clearance to mesh, a rack and pinion always has some backlash. It shows up as a small dead zone when the axis reverses. Common ways to reduce it include:

• Helical rack and pinion. More tooth contact gives smoother motion and less play.
• Dual pinion (electronic preload). Two motors drive two pinions on the same rack, and the controller applies opposing torque so one pinion always loads the opposite tooth flank.
• Spring-loaded pinion. A spring keeps the pinion pressed into the rack, at the cost of some wear.
• Correct mesh adjustment. Set the centre distance carefully; too tight causes heat and wear, too loose increases backlash.
• Backlash compensation in the controller. Useful for small residual play but it does not replace good mechanics.

Instalación y lubricación

• Ball screw alignment. Keep the screw parallel to the linear guides, and align both end bearings. Misalignment loads the nut sideways and shortens life.
• Do not remove the nut off the shaft. If it is needed, use a sleeve tool so the balls do not fall out.
• Keep dust out. Use bellows covers or wipers in dusty machines such as wood and stone routers.
• Lubricate. Grease the nut at regular intervals according to duty; an automatic unidad de lubricación keeps film consistent on production machines.
• Rack mounting. Align the rack straight and parallel to the guides, tighten bolts evenly and check mesh at several points along the stroke.
• Grease the rack teeth. Use a suitable grease and clean old grease and chips away before re-lubricating.

Resolución de problemas

Buying ball screws and racks from a China factory

Jinan Longqiao Electromechanical Equipment Co., Ltd is a factory in Shandong, China, supplying ball screws, racks and pinions, linear guides, servo systems and spindle motors for OEMs and distributors. Points buyers usually ask about:

• Cantidad del pedido y muestras: se acepta una cantidad mínima de pedido (MOQ) de 1 unidad y hay muestras disponibles.
• Custom work: ball screw length and end machining, and rack module, length, holes and surface treatment, can be made to your drawing.
• Registros de inspección: a machinery test report can be provided, and an outgoing-inspection video is available for racks.
• OEM: Se admiten requisitos de OEM.
• Shipping and payment: by DHL or sea freight, T/T and PayPal.
• Garantía y repuestos: las condiciones se confirman en su presupuesto para el modelo que solicite.

For an accurate quote, send the axis stroke, load, speed, accuracy grade and quantity, ideally with a drawing. You can also look at our fábrica y certificados pages, or browse the husillo de bolas y cremallera y piñón categories.