Transmission Chains: Short-Pitch Precision Roller Chains and Bushing Chains

Chain drives consist of the driving and driven sprockets mounted on parallel shafts, along with the chain loop that engages with these sprockets. The chain, acting as an intermediate flexible link, transmits motion and power between two or more parallel shafts or axes via the meshing of the chain with the sprockets.

This type of drive is known as a forced transmission with meshing properties. Among the most widely used transmission chains in industrial applications are short-pitch precision roller chains and bushing chains, commonly referred to as standard roller chains and bushing chains.

1.1 Roller Chain Composition

A roller chain typically consists of inner links, outer links, and connecting links (transition links).

a) The outer link plates of a typical roller chain have a figure-∞ shape. This design ensures that the cross-sectional strength of the chain plates is uniform, reducing the chain’s mass and inertial force during movement.

The pitch distance between the holes of the inner and outer link plates is controlled within strict tolerance ranges to ensure that the assembled chain maintains a consistent pitch and precise length tolerance.

b) The inner links, also known as roller chain links, consist of two inner link plates, two bushings, and two rollers. The hole of the inner link plate is tightly fitted with the bushing to prevent rotation within the plate’s hole. The inner hole and outer surface of the bushing are hardened and polished to ensure wear resistance.

The bushing’s internal and external diameters are tightly controlled within precise tolerances to ensure proper clearance and smooth rotation with the pin and roller. The rollers are hardened, smooth, and precisely dimensioned to ensure free 360-degree rotation and good engagement with the sprocket teeth. The height of each roller is designed so that it can rotate freely between the two chain plates.

c) The outer links, also called pin links, consist of two outer link plates and two pins. The center of the pin is tough enough to withstand impact, while its surface hardness is high to improve wear resistance. The pin is press-fitted into the holes of the outer link plates to prevent relative rotation, and the ends of the pins are riveted to secure them in place.

d) Connecting links (or transition links) are used to connect the two ends of the chain during assembly. Generally, the number of links in the chain is designed to be even to avoid the use of transition links. If an odd number of links is unavoidable, transition connecting links or compound transition connecting links should be used.

Compound transition links are considered better than simple transition links, which have a lower load capacity than the other chain links and should therefore be avoided when possible.

Connecting links are made by press-fitting two pins and riveting them onto one outer link plate. The hole in the second outer link plate (also called the connecting link plate) slides or lightly presses into the other end of the pin, making it easy to install and connect the chain. Two common locking methods for connecting links are elastic lock clips (also called circlips) and open pins.

1.2 Bushing Chains vs. Roller Chains

Compared to short-pitch precision roller chains, short-pitch precision bushing chains do not feature rollers. In these chains, the bushings directly engage with the sprocket teeth. This design enhances shock resistance and chain link precision, making bushing chains more advantageous in high-speed applications, such as timing chains used in automotive engines.

1.3 Multi-Row Roller Chains

When two or more single-row roller chains are arranged in parallel and connected by long pins, they form multi-row roller chains. The more rows the chain has, the higher its load capacity. However, manufacturing and installation precision requirements also increase, and it becomes more challenging to ensure uniform load distribution across all rows, which could reduce the service life of multi-row chains. Therefore, the number of rows should not be excessive. According to GB/T 1243—2006, double-row and triple-row roller chains are specified, with size parameters and technical specifications for these two types. For high-power transmission, two or more double-row or triple-row roller chains may be used.

While GB/T 1243—2006 only includes double-row and triple-row roller chains, manufacturers actually produce multi-row roller chains in a variety of configurations. Commonly used versions include 4–6 rows, and large-sized chains may even have up to 8 rows. For example, roller chains used in petroleum drilling rigs are often multi-row chains.

1.4 Chain Standards and Specifications

GB/T 1243—2006 covers a broad range of applications for short-pitch precision roller chains and bushing chains. The standard includes three main categories: standard A and B series chains and ANSI heavy-duty series chains. The pitch specifications range from 6.35 mm to 114.30 mm (with bushing chains having a pitch size ≤9.525 mm and roller chains having a pitch size ≥8.00 mm, and heavy-duty roller chains ranging from 19.05 mm to 76.20 mm).

• A Series: Derived from the ANSI standard, typically marked with an “A” suffix, with 14 different chain numbers (including 04C, 06C, and 085).
• B Series: Originating from Europe, typically marked with a “B” suffix, and containing 18 different chain numbers (including 081, 083, and 084).
• ANSI Heavy-Duty Series: Marked with an “H” suffix, with 9 different chain numbers.

For all these chain types, the basic parameters and key dimensions of the chains and the matching sprockets are specified in GB/T 1243—2006. These specifications ensure complete interchangeability of chains from different manufacturers, and allow for interchangeable individual chain links.

1.5 Automotive Industry Chain Standards

The recently developed mechanical industry standard, “Roller Chains and Bushing Chains for Automotive Applications (Draft),” focuses on small-pitch (≤9.525 mm) precision roller chains and bushing chains specifically used in automotive engines. These chains are typically used in timing mechanisms within automotive engines, such as camshafts and drive systems. The standard specifies the main structural types, dimensional parameters, and performance requirements for these chains.

Automotive chains, unlike regular industrial chains, must perform under extreme conditions such as high-speed operation, high temperatures, and alternating loads (idle speed, acceleration, deceleration). As a result, automotive chains have higher demands for reliability, dimensional accuracy, and wear resistance. The “Automotive Roller Chains and Bushing Chains” standard thus has stricter requirements than GB/T 1243—2006 for regular precision roller chains and bushing chains.

1.6 Fatigue Resistance of Chains

Chains are subject to alternating loads during operation. Under heavy loads, fatigue failure of the chain plates is a common issue. The ANSI heavy-duty roller chains feature thicker chain plates compared to standard roller chains. This increased thickness improves the fatigue strength of the chain, making it more suitable for medium-to-low-speed, heavy-load applications. Consequently, heavy-duty series chains have better fatigue resistance and are more suitable for low-speed, heavy-load transmission conditions.