TTEECHCHNINICALCAL EN ENGGINEEINEERRININGG 89 www.diamondchain.com TECHNICAL ENGINEERING General Drive Considerations One of the main advantages of the roller chain drive is its ability to perform well under widely varying conditions. Despite this ability, there are a number of rules of good design practice which, if considered early in the design pro- cess, will enable the user to obtain desirable results. Basic dimensions and minimum ultimate tensile requirements for single-pitch, double-pitch and attachment roller chains are specified by various standards organizations worldwide. ASME/ANSI, The American Society of Mechanical Engineers and The American National Standards Institute, defines dimensions such as: pitch, roller width, roller diameter, link plate height, link plate thickness and pin diameter. The primary purpose of the standard is to ensure that manufacturers will produce chains and sub-assemblies that are similar dimensionally and therefore interchangeable. In addition, the standard does offer the user some assurance of quality by defining a minimum ultimate tensile strength for each model of chain. However, tensile strength is not always a valid method to differentiate one manufacturer’s product from another. It is very important to remember that dimensional standardization does not define quality or performance characteristics. Minimum Ultimate Tensile Strength: Minimum Ultimate Tensile Strength, MUTS, is the static load required to break the chain. Tensile strength values shown in this catalog are not allowable working loads. Load or tension applied 1 to the chain in service should never exceed ⁄6 th of the UTS. If exceeding this value is necessary for a specific applica- tion, contact Diamond Chain. Warning! A roller chain should never be loaded above 50% of MUTS for even one cycle. Doing so will permanently damage the chain. Allowable Working Load: Roller chains with equal tensile strengths can have very different working load capacities. Contrary to popular belief, there is no consistent relationship between a roller chain’s working load capacity and its ulti- mate tensile strength. A chain with a higher tensile strength than a Diamond chain could have a much lower working load capacity. Selecting Chain Size: There may be several suitable selections for any particular application. Loads, speeds, envi- ronment, cost, required service life or other factors will determine the final selection. Generally, the lowest cost drive will consist of a single strand chain of the smallest pitch that can accommodate the load. The speed and number of teeth of the smallest sprocket, most commonly the driver sprocket, also have an effect on the selection of chain size. As a rule, the smaller the pitch the higher the permissible operating speed. Selecting Sprockets Small Sprocket: The smallest sprocket is usually the driver or input sprocket. As the chain enters and exits, it rises and falls as each pitch engages and disengages the sprockets. % SPEED VARIATION 14 12 10 8 6 4 2 0 6810 12 14 16 18 20 22 24 26 28 30 NUMBER OF SPROCKET TEETH This movement, called chordal action, causes chain speed variations (drive roughness) that may be objectionable in some applications. These speed variations can normally be minimized by increasing the size of the sprockets, as shown. 90 TECHNICAL ENGINEERING General Drive Considerations 317-638-6431 1-800-872-4246 To minimize the negative effects of chordal action, the following are suggested guidelines for the minimum number of teeth in the smallest sprocket: Slow Speed (Type A* lubrication region) 12 Teeth Medium Speed (Type B* lubrication region) 17 Teeth High Speed (Type C* lubrication region) 25 Teeth * More detail on type A, B and C lubrication can be found in the Roller Chain Lubrication section of this guide. Hardened Teeth: Tooth loading increases as the number of teeth in the sprocket decreases. Hardening of sprocket teeth is recommended when the number of teeth is 25 or less and/or the sprocket will operate in: 1. Drives that are heavily loaded. 2. Abrasive conditions. 3. High speed drives. 4. Drives requiring extremely long life. Chain Wrap: The recommended minimum wrap angle on the smallest sprocket in the drive is 120°. Wrap angle can be reduced to 90°, if good chain tension adjustment is maintained. If chain tension is not closely maintained with less than 120° wrap, the chain can jump teeth, resulting in damage to itself and/or the sprocket. Note: For a ratio of 3:1 or less there will always be 120° or more wrap on the small sprocket, regardless of the center distance. Drive Ratio: The ratio of the sprocket sizes is determined by the desired speed reduction or increase. The maximum recommended ratio for a single reduction is 7:1. In practice, the practical single reduction limit is affected by: the mini- mum size of the small sprocket, the maximum size of the large sprocket, and the need for sufficient wrap on the small sprocket. It is possible to utilize a reduction as great as 9:1 but a double reduction is preferable. It is important to remember that drive ratio is a function of the number of teeth on the sprockets, not the sprockets’ diameters. 7:1 RATIO 7:1 RATIO (TWO DRIVES) 91 www.diamondchain.com TECHNICAL ENGINEERING General Drive Considerations Drive Arrangements Shown below are recommended, acceptable, and not recommended drive arrangements, along with preferred direction of travel. Every effort should be made to utilize the recommended or acceptable layouts in order to obtain optimum drive life. RECOMMENDED ACCEPTABLE NOT RECOMMENDED Chain Length: Chain length must be an integral number of pitches (no fractions of pitches). Additionally, every attempt should be made during the design process to define a chain length, which is an even number of pitches including the connecting link. In a fixed center-distance drive this can be done by selecting sprockets that provide a ratio near that desired. In an adjustable center-distance drive this is achieved by providing sufficient adjustment or “take-up” so that an even number of pitches can be used and still operate with proper tension. If neither of the above conditions can be met, a chain having an odd number of pitches is required. These designs require the use of offset links or “half links.” Offset links are generally costly and will significantly reduce the chain’s load carrying capacity. Offset Links If required, Diamond offers two types of offsets: single-pitch and multiple-pitch. Single-pitch offsets are constructed using hybrid link plates consisting of half pin link plate and half roller link plate contours. Single-pitch offsets are secured within the chain using a slip-fit pin and cotter keys. Note: Single-pitch offsets can reduce the load capacity of a roller chain by as much as 30%. 92 TECHNICAL ENGINEERING General Drive Considerations 317-638-6431 1-800-872-4246 Multiple-pitch offsets, commonly two pitches in length, are constructed with the same basic design as a single-pitch offset, with the exception that the offset link itself is riveted together with a standard roller link assembly. Multiple-pitch offsets afford the user superior performance and generally are less costly than single-pitch offsets. However, multiple-pitch offsets still reduce the load carrying capacity of the chain. Note: Multiple-pitch offsets can offer virtually the same integrity as the base chain. However, some reduction in load carrying capacity can result from their use. Connecting Links Connecting links are used to join the ends of the chain together once installed on the drive. Diamond offers two types of cover plates depending upon the application and/or the user’s preference: slip-fit or press-fit. Slip-fit cover sides are supplied when the user prefers ease of assembly and disassembly. The cover plate of a slip-fit connecting link has pitch holes that are larger in diameter than the pins. This allows the user to “slip” the cover plate onto the pins before installing a spring clip or cotters. This style of connecting link is inherently weaker than the base chain because its slip-fit construction does not have the same integrity found in the assembled chain. Note: Slip-fit connecting links can reduce the chain’s working load capacity by as much as 30%. Press-fit cover plates are provided when the integrity of the connecting link needs to be equal to that of the base chain. In this design, the cover plate has pitch holes that are smaller in diameter than the pins. This requires the user to “press” the cover plate onto the pins before installing a spring clip or cotters. While more difficult to install, these links do provide the greatest load carrying capability. Diamond does not provide any specific tool for use with the installation of a press-fit cover plate. However, a modified C-Clamp-type device often makes the job much easier. Note: Never drill out or enlarge the pitch holes of a press-fit connecting link cover side to make the installation easier. “C-CLAMP”-TYPE DEVICE 93 www.diamondchain.com TECHNICAL ENGINEERING General Drive Considerations Chain Tensioning/Length Adjustment: Proper chain tension is critical to achieving acceptable service life. Chain tensioning may be accomplished by either: adjusting one of the shafts to increase the center distance, using a movable idler sprocket, or removing pitches from the chain to compensate for wear elongation. For the majority of slow and medium speed chain drives, the total mid-span movement in the slack span should be approximately 4-6% of the drive’s center distance. For drives operating at high speeds, impulse or reversing loads, the total movement should be reduced to 2-3% of the center distance. Drives with vertical centers should also be adjusted to the smaller percentage.