The Coupling Handbook: Part V

Popular Lubricated Metallic Coupling Types - Grid & Chain 

The Grid coupling was designed in 1919 and has since found favor in applications involving pumps, gear boxes, cranes, mixers/agitators, bulk material handling systems, mining equipment, compressors, and fans/blowers. Grid couplings are most popular in the Steel, Pulp & Paper and Mining Industries.

This design is characterized by two flanged hubs with slots cut axially into the perimeter of their flanges, forming a ring of narrow, closely set teeth around each flange. When the hubs are brought together, their slots match, and a single serpentine grid of spring steel is wrapped around both flanges so that the spring loops back and forth between the two hubs, nesting in the slots. Lubrication is required, so a collar-type cover with seals and gaskets is used to hold the lubricating grease in place. It also keeps the grid spring from migrating out of the hub slots when misaligned. Because the covers can be removed, worn or broken grids can be replaced easily without disturbing the positions of either driver or driven equipment. Two cover designs will be described later.

Torque is transmitted from one hub to another through the grid spring. The flexible nature of the grid absorbs impact energy by spreading it out over time, thus reducing the magnitude of peak loads. This is possible because of the progressive contact that occurs between the flexible grid and the curved profile (axial crowning) of the sides of the hub teeth. As the load increases, more of the tooth comes into contact with the grid, allowing the coupling to handle shock loads that occur within the system.

The Grid coupling teeth do not mesh between the hubs, therefore, if the grid spring were to fail the coupling would no longer transmit torque.

The most common type of driver is an electric motor. However, the ability of the grid coupling to damp vibrations allows it to be used with reciprocating engines (4 or more cylinders), as long as the proper service factors are applied in sizing the coupling.

Standard grid couplings can be used in vertical-axis drives without any modifications. Many modifications or adaptations are possible for special applications. Examples of the variations of grid couplings that are possible include floating shaft designs, disc brake couplings, spacer couplings, high speed applications, engine flywheel adapters and clutches.

The grid coupling typically competes with either elastomeric or gear couplings. Due to its high torque-to-outside diameter ratio the grid coupling may be used in applications where elastomeric couplings are too large to fit into the space constraints. It also becomes a less expensive alternative where torque levels start to require comparably rated elastomeric couplings to be much larger. The downside is that the grid coupling tends to be higher maintenance than an elastomeric type coupling due to its lubrication requirement, lesser misalignment capacity, and installation time.

The grid coupling excels in applications where an all-metal coupling is desired but moderate vibration damping is required. The resilience of the grid gives this design a damping capability that is typically not available with an all-metal coupling.

Most designs have backlash or free play between the fit of the grid teeth and spring and are not suited for motion control applications. Temperature capacity is usually no greater than 220°F (120°C), due to the rubber seals, which keeps them out of some applications.

To the users advantage, grid coupling manufacturers have kept their couplings directly interchangeable. For the most part, components of one can be used with another. The only real difference between manufacturers is the shape of the seal and gaskets. Each manufacturer uses a different shape of seal and gasket that fits with their cover assembly. Therefore, it is required that you use the corresponding seal and gasket of the cover manufacturer. If a different manufacturers seal and gasket are used, there is the possibility that a seal will not form and there could be loss of lubrication.

Being an all-metal coupling the only purpose of the cover is to hold the necessary grease in place to provide proper lubrication. Some manufacturers shot-blast the tooth area of the hubs to remove any burrs and sharp edges that may cut into the grid spring. This allows for a smoother transmission from the grid teeth to the grid spring.

Torque Transmission and Torsional Flex

Accommodation of Misalignment and Axial Displacement

Angular and parallel misalignment are restricted by the design of the covers and seals, and this restricted misalignment will introduce fairly significant reactionary loads on the shafts.
Parallel: The movement of the grid in the hub grooves accommodates parallel misalignment and still allows full functioning of the grid-groove action in damping out shock and vibration. Maximum parallel misalignment ranges from 0.012" to 0.022" depending on the size of the coupling.

Angular: Under angular misalignment, the grid-groove design permits a rocking and sliding action of the grid and hubs without any loss of power through the resilient grid. Maximum angular misalignment is ¼°.

Axial: End float is permitted for both driving and driven members because the grid slides freely in the grooves. Maximum allowable end float ranges from 0.198" to 0.571" depending on the size of the coupling.

Grid Coupling Types

There are two designs for the grid coupling: tapered grid and straight grid. The straight grid design is the original style developed in 1919. In the 1950's, the design was enhanced with a tapered grid, which supports higher torque ratings than comparably sized straight grids. This is due to the sliding action of the grid spring on the hub grooves versus the twisting action of the straight grid. Additionally, the hub teeth of the tapered grid style are stronger due to the base radii of the teeth.

Cover Styles

The grid coupling is available with two different cover styles: horizontal and vertical. The horizontal design splits the cover axially so it can be installed or removed and replaced externally, in a wrap-around fashion.The two halves are joined by bolts inserted in tangential orientation. The vertical design splits the covers radially into two flanged circular halves, with one half pre-fitted over each hub from the shaft end and joined by bolts oriented axially around the perimeter of each half. Both cover styles allow the coupling to be opened for service or grid replacement without disturbing the installation of driver or driven equipment.

Horizontal Cover Grid Coupling - Lovejoy, Inc.
Horizontal Cover Grid Coupling
The horizontal cover is manufactured from die-cast aluminum. Do to it having a smaller outside diameter than the vertical cover for a comparably sized coupling, it is ideal for limited space applications. Also, it is well suited for reversing applications due to a lug that is molded into the inside of the cover. The lug fits in between the grid spring and does not allow the cover to spin in the opposite direction of the hubs. If the cover were to spin it would break the seal between the seal and the hub and the lubrication could leak out. Horizontal covers are always required for spacer couplings.

Vertical Cover Grid Coupling - Lovejoy, Inc.
Vertical Cover Grid Coupling
The vertical cover is manufactured from stamped steel. It can be used in higher speed applications than the horizontal cover because of the cover shape. Vertical covers cannot be completely removed without demounting the hubs, but for simple grid maintenance they are typically just moved back over the hubs. 

Adequate clearance must be available to do so. Vertical covers do not have the lug like the horizontal version, so reversing applications can cause the covers to spin in the opposite direction of the hubs. The horizontal cover is typically the more popular cover. It is believed that this has to do with installation and maintenance. When using the vertical covers, the covers have to be put on the shafts before the gaskets and hubs are installed. It is easy to overlook this installation sequence and force installed gaskets and hubs to be taken off again so the cover halves can be put in place. The horizontal cover avoids this mistake by allowing installation after the hubs and gaskets are in place.

Spacer Type

Spacer Grid Coupling - Lovejoy, Inc.
Spacer Grid Coupling

The pump industry (primarily ANSI chemical pumps) has long desired spacer couplings for ease of maintenance. Spacer design couplings allow for a standardized gap between the ends of the driver and driven shafts. The spacer allows the coupling to be opened up with a gap wide enough to let the pump casing be removed from the "wet end" of the pump for servicing of components such as the impeller, seals, bearings, etc. without disturbing motor or pump mountings.

The grid spacer coupling meets standard ANSI spacing requirements for pump disassembly. One of the benefits of the grid spacer coupling is that various components can be mixed/matched in combinations to achieve dozens of other shaft separations beyond the ANSI standards of 3 ½", 5" and 7".

The grid spacer coupling is achieved by using a shaft hub that is bolted to a spacer hub. There are usually three or more lengths of spacer hubs available in each size coupling. The shaft hub has the finished bore and keyway for the driver/driven shaft. The grid spring wraps around the grid teeth of the spacer hub. The horizontal cover fits over the spacer hubs. Each half of the full spacer coupling uses a spacer hub/shaft hub combination.

Half-spacer couplings are made possible by using a standard hub on one side and the spacer hub/shaft hub combination on the other side. The spacer hub is bolted to the shaft hub with four to twelve hex head cap screws depending on the size of the coupling. By removing these screws, the center section of the coupling can be dropped out.

Grid Spring

The grid springs are made of a high tensile alloy steel that is formed to shape, then hardened and tempered under controlled conditions. The grids are then shot-peened, compressing the surface molecules and leaving a residually stressed surface. This process creates a stronger surface in compression.

Examples of Failures

There are three main causes of grid spring failure. The first, mode of failure, has to do with misalignment. When the grid coupling is misaligned beyond the specified catalog limits, the grid seal will fail, causing the lubrication to leak out. This loss of lubrication will cause the grid spring to fail at the curves of the grid. This is not a catastrophic failure and the grid coupling will continue to transmit torque. The user may not even realize that the grid spring has failed until the next time maintenance is done on the coupling.

The second mode of failure mimics the first mode, but occurs even in correctly installed and aligned couplings if not given proper and sufficient lubrication. When this situation occurs, the grid spring will fail at the curves of the grid. This is not a catastrophic failure and the grid coupling will continue to transmit torque. The user may not even realize that the grid spring has failed until they do maintenance on the coupling.

The third mode of failure has to do with an over-torque situation. If the driving equipment transmits more torque than the coupling can handle, the grid spring will fail at the center of the grid. When this occurs, the grid coupling will no longer transmit torque from the driving to driven equipment. The grid spring would need to be replaced immediately in order to continue operation.

Routine Maintenance

Adequate lubrication is essential to prolong the life of the grid coupling and to obtain trouble free service. Grid coupling manufacturers specify that the couplings be re-lubed annually when using a common industrial lubricant. Special lubrication can be used to extend lube intervals. A coupling that is exposed to extreme temperatures, excessive moisture, frequent reversals or grease leakage may require more frequent lubrication.
It is recommended that the coupling covers be removed to check lubrication condition, alignment and the general condition of the grid and teeth every year. Couplings used in ambient temperatures greater than 158ºF, at high speeds and/or frequent reversing applications may require more frequent inspection, re-lubing and possible grid replacement.

Special Designs

Some manufacturers offer further adaptations of the grid coupling for special applications. These include; controlled torque, disc brake, brakewheel, engine flywheel, piloted, and high-speed designs.

Chain Couplings

The chain coupling is used extensively on unsophisticated applications such as agricultural equipment and machinery because it is an all metal, rugged, lightweight and economical method for connecting two shafts.

The coupling consists of two sprockets (hubs with chain-matching teeth on the periphery) connected with a double roller chain. It is easy to install, easy to maintain and easy to rough align. The chain coupling allows quick shaft disconnection by removal of the chain. A cover is used to contain grease and keep out dirt. The coupling can be supplied with taper bore bushings for easier installation, and can be upgraded with hardened sprocket teeth and a precision chain to improve wear life. The chain is the replacement element although the sprocket may also wear.

Misalignment is accommodated by the loose fit of the chain to the sprockets. Therefore, the coupling is a wearable unit. Couplings of this type are suitable for a maximum of 700 HP at 1800 RPM, or 145 HP at 3600 RPM. The maximum bore in the largest size is 4 inches and maximum angular misalignment is ½°. Typically, their applications present smaller values.

Go To Next Section - Part 6: Popular Non-Lubricated Metallic Coupling Types - Disc, Diaphragm, Spring, 3 Link, & Beam
Go Back To Handbook Index

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