Rectangle

所属分类: Mandrel

Rectangular splines are an early type of multi-tooth drive connection between the screw core and screw elements in co-rotating twin-screw extruders. The tooth profile is a straight rectangle, following standard GB/T1144, commonly with 6, 8, or 10 teeth, and they’re centered with a small diameter. Compared to the old single key design, the multi-tooth can share torque better and increase load capacity. Compared to involute splines, they’re easier to machine and cheaper, so they were often used in older domestic medium-to-low torque twin-screw machines. Nowadays, new machines mostly don’t use them, and they’re mainly for replacing parts in older equipment.

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Detailed description

Technical Details of Rectangular Spline

Structural Geometry

• External spline (core shaft): Multiple straight-sided rectangular external teeth are machined on the outer circle of the core shaft.

• Internal spline (screw element): Matching rectangular keyways are machined on the inner bore of conveying, kneading and mixing elements.

• Minor-diameter centering. Torque is transmitted through tooth flanks. Axial positioning and locking of elements are realized by lock nuts and retaining rings.

• Comparison with single key: A single key transmits torque via only one key; rectangular spline carries load by multiple teeth simultaneously, delivering remarkably higher torque capacity.

• Comparison with involute spline: Involute spline features curved teeth with high root strength and self-centering under load. Rectangular spline adopts straight teeth without self-centering function and suffers greater stress concentration at tooth root. Tooth flank wear and growing clearance tend to occur under high-speed and heavy-load conditions.

• Model marking example: 6×23×26×6 GB/T1144, internal bore rectangular spline.

• Assembly requirement: Spline grease must be applied during assembly and disassembly; dry friction is prohibited.

Material Configuration

• Core shaft: 40CrNiMoA, quenched & tempered + tooth surface hardening.

• Screw element body: 38CrMoAlA, WR5, WR13.

• Spline tooth flank grinding to improve wear resistance.

Manufacturing Process

• Forging and quenching & tempering of core shaft, followed by milling of rectangular external teeth.

• Broaching or grinding of rectangular internal spline on screw element blanks.

• Dimensional control for minor-diameter centering.

• Disadvantage: Uniform load distribution among multiple teeth is difficult to guarantee.

Process Parameter Characteristics

• Torque capacity: Medium. Higher than single key and lower than involute spline of the same specification. Not suitable for high specific torque main extruders.

• Centering accuracy: Moderate. Tooth flank wear under long-term alternating load gradually increases circumferential clearance and causes phase offset.

• Interchangeability: Moderate. Poor cross-machine interchangeability for parts of the same specification.

• Assembly & disassembly: Elements can slide axially for mounting and removal; grease application is mandatory.

• Failure modes: Tooth flank wear and crushing; tooth breakage rarely occurs. Clearance after wear cannot be repaired.

Functions of Rectangular Spline

•Transmit rotational torque. Multiple teeth mesh simultaneously to transfer torque output from gearbox through core shaft to all screw elements. It withstands alternating torque under compounding conditions and outperforms single key structure in load capacity.

•Enable modular screw assembly. Screw elements can slide axially along the core shaft. Elements can be added, removed or rearranged freely to flexibly adjust screw process configuration.