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Selecting Couplings For Large Loads

Selecting a coupling type for any drive application requires not only consideration of design concerns, but also other factors related to maintenance, size and cost. Depending on a person's area of concern, some of these may be easily overlooked.

Most engineers consider design parameters, such as torque rating, service factors, speed, misalignment and bore size, in selecting couplings. But others who influence selection have different priorities. Purchasing agents are concerned about price, delivery and vendor support. Production or maintenance personnel give high priority to reliability, ease of installation and maintenance costs.

To illustrate the many factors to consider in choosing couplings, we selected a bulk-material-handling belt conveyor application. In this example, a 150 hp (112 kW) motor operating at 1,750 rpm drives a double-reduction parallel-shaft gear drive with an output speed of 84 rpm. Couplings must be used to connect the shafts between motor and gear drive (high-speed shaft) and between the gear drive and the conveyor (low-speed shaft). (Figure 1) The example considers four types of flexible couplings commonly used in conveyor applications: grid, gear, elastomeric, and disc.

Figure 1: Belt conveyor drives often consists of (right to left) motor, gear drive and belt pulley (inside housing). Here, an elastomeric coupling connects the motor to the gear drive (high-speed shaft), while a grid coupling connects the gear drive and belt pulley (low-speed shaft)

 

Table 1 lists the selection factors and coupling options, which are described in the following sections. Values shown for the different parameters (torque, service factor, etc.) are typical, but may vary with different models and manufacturers.

Though the example focuses on conveyors and specific coupling types, the same selection method applies to other high-torque applications and couplings.

Table 1 -- Coupling selection factors for belt conveyor

Torque required: 5,400 lb.-in. (high-speed side), 112,500 lb.-in. (low-speed side).

Shaft sizes: motor 2-3/8 in., gear drive (high-speed side) 2-1/4 in., gear drive (low-speed side) 4-1/2 in., belt drive headshaft 5-7/16-in.

Couplings listed meet torque and shaft size requirements.

Design Considerations

This section overviews how each design factor listed in Table 1 influences coupling selection. Cost and maintenance factors are reviewed later.

Torque Rating

One of the key factors in selecting a coupling is its torque rating, in other words the amount of torque it can transmit. Another factor, also important, is the amount of torque it can transmit in a given size. This is called the torque density, sometimes referred to as power density, which is defined as torque rating divided by outside diameter. (Table 2)

Gear couplings pack the most torque capability in a small size. However, the maximum bore size of gear couplings often limits their selection. After gear couplings, other couplings with metallic flexible elements, such as grid or disc, offer the most torque for their size. The elastomeric couplings considered in this example are of the rubber tire type that is loaded in shear. These couplings offer the least torque density.

Service Factor

Once the torque requirement has been determined for normal operating conditions, you need to increase the selection torque requirement to accommodate torque fluctuations in the particular application. To do this, engineers apply a service factor (SF), usually larger than 1.0, which indicates the perceived severity of the service. Higher numbers indicate more severity. However, no rating standard or standard set of operating conditions, other than 1.0, is for smooth load systems.

Unfortunately, coupling manufacturers do not agree on these values. Each manufacturer has developed its own SF values based on experience. The manufacturers' values also vary with the coupling materials, which range from carbon steel to elastomers and composite materials.

Almost all manufacturers rate their couplings for peak overloads of 200 percent of the catalog rating to accommodate motor start-up loads. But, ultimate strength varies greatly among different coupling types and different brands. This variation often depends on the coupling materials.

To avoid the confusion of these different ratings, select coupling types that are field-proven in your type of service and recommended by the coupling manufacturer.

Outside Diameter

Large coupling diameters and long hub lengths often cause interference with base plates, piping, shaft fans and coupling guards.

Below 50 hp (37 kW) capacity, the four coupling types have similar diameters. But, as torque and shaft size increases, couplings with metallic members (grid, gear and disc) have smaller outside diameters than elastomeric types. This is particularly evident in the article's application example, where the elastomeric coupling for the low-speed shaft is twice the diameter (24-in., 61-cm) of the metallic couplings.

Weight

At 674 lbs. (306 kg), the elastomeric coupling for the low-speed shaft weighs 500 lbs. (227 kg) more than a comparable gear or disc coupling. Such weights may induce deflections in the shafts of the connected equipment, and can cause vibration. Therefore, check the drive for the effect of such loading on the shaft and bearings.

Coupling Moment of Inertia

Where conveyor applications require controlled acceleration and deceleration, design engineers use coupling inertia values (wr²) to properly size motors for start-ups and brakes for stopping. However, for belt conveyors that usually have long acceleration and deceleration times, coupling inertia is seldom a problem.

Torsional Deflection

As torque is transmitted through a coupling, its flexible element deflects between the two hubs, a condition known as torsional deflection or windup. Some torsional deflection is normally desirable, as it cushions uneven torque loads, thereby saving wear and tear of the connected equipment.

Torsional deflection in the grid coupling of this example lets the shafts rotate ½ to ¾ degrees relative to each other, whereas the torsionally soft elastomeric couplings allow 5 ½ to 6 degrees. Gear and disc couplings have negligible windup.

Torsional Stiffness

The resistance of a coupling to torsional deflection, called torsional stiffness, affects the critical speed of the system. Designers often overlook this factor for conveyor applications. But, the effect of torsional stiffness values on critical speeds and vibration should be evaluated.

Gear couplings offer the highest torsional stiffness, and elastomeric couplings the lowest. Grid and most elastomeric couplings get progressively stiffer as the applied torque increases in a given size coupling.

Backlash

Rotational clearances between coupling parts allow for another type of rotation, called backlash. Gear couplings contain a small amount of this clearance between hub teeth and sleeve teeth. In grid couplings, the clearance occurs between the grid-member and hub slots. Clearance is required for misalignment accommodation and provides space for a lubrication film.

A disc coupling has no backlash because its components are tightly held together. Some types of elastomeric couplings have minimum backlash.

Misalignment Capacity

Coupling manufacturers offer widely varying recommendations on allowable shaft misalignment. The suggested operating limits outlined in Table 1 allow for simultaneous extremes of offset and angular misalignment. Falk's experience shows that exceeding these limits increases loads on both the coupling and its connected equipment, and can reduce their service lives. Some coupling manufacturers publish higher values that allow more angular misalignment if there is no offset misalignment and vice versa.

Manufacturers also give suggested installation and static limits. Installation limits are smaller than operating limits to allow for dynamic movement of equipment and settling of foundations. Static limits apply to non-rotational conditions. For example, removing paper rolls from a paper machine (static condition) may require more angular misalignment than operating conditions.

Be sure you know whether the coupling manufacturer is giving you installation, operating or static design limits. Often, these three sets of values are poorly labeled in sales literature, leading to reader confusion.

Grid Pattern: grid couplings combine high load capacity with torsional flexibility

The four coupling types vary in their ability to accommodate shaft misalignment. Shear type elastomeric couplings typically handle the most misalignment.

Within the metallic coupling types, gear couplings have the most misalignment capability, followed by disc and grid couplings.

Shaft Gaps

Grid and gear couplings permit the assembly of equipment with the smallest shaft gaps, or the distance between shaft ends, an important factor where space is limited. Close-coupled disc couplings are not available for high-torque, low-speed applications. However, a recently developed disc coupling, (Figure 5), offers the same gap as grid and gear types for most motor shaft (high-speed) applications (listed in Table 1).

Shear Deal: elastomeric couplings (shear type) give torsional flexibility and require no lubrication.

A shear-type elastomeric coupling requires larger shaft separation to accommodate its flexing element. This gap typically ranges from 1-in. (2.54-cm) on a small coupling to over 5-in. (12.7-cm) on a large one.

Balance

Coupling unbalance can cause vibration in the connected equipment. Its AGMA balance class expresses the amount of coupling unbalance, where higher numbers indicate better balance and smoother operation. Most gear and disc couplings can be balanced by the coupling manufacturer to improve their balance class rating and operating speed range. Based on Rexnord's experience, conveyor operating speeds are generally low enough, so it is not necessary to balance the couplings.

Other Considerations

Design considerations are not the only important factors to consider when selecting the proper couplings. Other selection factors to consider relate to cost, maintenance and environmental conditions.

Cost

Initial Cost: Grid couplings generally cost the least for shafts through 4-in. (10.2-cm) diameter. Beyond this point, the high-torque capacity per size of gear couplings makes them the least expensive.

Elastomeric couplings are inexpensive in fractional to low-horsepower sizes, but their cost grows rapidly as torque and shaft sizes increase. In this example, for the high-speed shaft, elastomeric or disc couplings cost USD$200 more than grid or gear couplings. For the low-speed shaft, the least expensive cost is the gear coupling, followed in expense by grid, disc and elastomeric couplings. In this example, the elastomeric coupling costs USD$1,200 or more than the other types.

In addition to the purchase price, other costs are incurred for replacement parts and downtime. Replacement Costs: OEMs often supply the lowest cost couplings on their equipment to minimize total equipment cost. Unfortunately, the lowest cost coupling is often not the best choice for the application and causes more expense after installation.

This situation is evident when considering what parts of a coupling typically wear out and how difficult it is to replace these parts. In a gear coupling, the teeth generally wear out, which requires a completely new coupling. Therefore, the replacement cost usually erases any initial cost savings.

The other three coupling types - grid, elastomeric and disc - only require the replacement of the less costly flexible elements. The cost of a replacement grid is usually well below that for an elastomeric or disc element. This makes the grid coupling a better value for the low-speed shaft, even though its initial price is higher than a gear coupling. Downtime: A conveyor shutdown caused by coupling failure can easily cost thousands of dollars per hour. The problem is compounded if the failed coupling is difficult to service.

Gear couplings, which must be replaced entirely, are the most difficult to service. Replacement typically requires moving the connected equipment, then removing the hubs. New hubs are then installed, and the equipment must be repositioned and realigned. This is not an easy task, for example, when working on a confined conveyor drive platform 50 ft. (15 m) above ground.

When a grid coupling fails, the grid usually fails in fatigue due to excessive misalignment or torsional vibration. The coupling can continue operating until several segments are broken. Grids can be replaced without moving the connected equipment.

Disc couplings are designed such that they rarely fail due to fatigue, if operated within their misalignment limits. With disc couplings, the disc usually fractures due to improper bolt tightening or excessive misalignment. Unitized disc packs, wherein discs, bushings and washers are held together in a sandwich, simplify replacement and avoid lost components.

Elastomeric flexing elements experience fatigue failures due to excessive misalignment, as well as overloads and environmental deterioration. Their flexing elements are usually easy to replace.

Maintenance Interval

Until recently, grid couplings had to be lubricated annually to replace grease in which oil separated from the thickeners. A new type of long-term grease (LTG) extends this interval to five years.

When applied to gear couplings, LTG grease extends the interval from six months to three years. Gear couplings depend more on lubrication than grid couplings because of their higher tooth contact stress. Up to 90 percent of gear coupling failures are lubricant related, such as lack of lubricant, leakage, contamination, wrong grade or lubricant breakdown under misalignment conditions.

Disc and elastomer couplings do not require lubrication. Moreover, disc couplings can be inspected, while rotating, with a strobe light. Cracks in the disc assembly are an early sign of impending failure.

Environmental Factors

Bulk material conveyors operating outdoors expose couplings to temperature extremes plus sunlight, ozone, moisture and abrasive contaminants.

Disc couplings, which have neither seals nor lubricants, offer the largest temperature range and are unaffected by most environmental conditions found in conveying.

Grid and gear couplings offer moderate temperature ranges, which are limited by seals and grease. Grid couplings tend to be more forgiving of abuse and less sensitive to contaminants, compared to gear couplings.

Elastomeric couplings have the smallest temperature range. At temperatures approaching -40o F (-40o C), they get stiff and brittle; above 150o F (66o C), the heat may degrade the elastomeric element. If either of these conditions is common in an application, it could shorten the elastomeric element fatigue life. Ozone and sunlight also may deteriorate elastomeric compounds.

Making the Choice

For this particular conveyor application example, we selected grid couplings for both the high-speed and low-speed shaft connections. This coupling is the most economical choice based on total costs. It has a low initial cost, the lowest replacement parts cost and requires little maintenance. It also provides adequate misalignment capacity, gives some resilience for vibration damping, and is not limited by environmental factors.

Conclusion

Selecting a coupling type for a drive application requires consideration of design elements, such as torque rating, service factor, weight, torsional deflection and stiffness, backlash and misalignment capacity, as well as cost, maintenance and operating conditions. By evaluating all of the advantages and disadvantages of each coupling type in a particular application, the proper coupling selection - whether grid, gear, elastomeric or disc - will be evident.