How sensitive are Grid Couplings to misalignment?

Grid couplings are very sensitive to misalignment. They are designed to handle almost no parallel shaft misalignment, and only minimal angular misalignment (as called out in the Lovejoy grid coupling rating chart and the coupling pre-selection guide below).

While misalignment must be considered and proactively addresses when using a grid coupling (see Why Lovejoy Offers Shaft Alignment Dial Indicator Kits), there are several major benefits to using a grid coupling. The biggest is that it is an all metallic flexible coupling design that can transmit significant torque in a small footprint (high power density) while also (unlike metallic gear & disc couplings) providing system vibration dampening capability. Grid couplings are a well proven technology, and are readily available from stock from a few leading coupling brands (inclusive of Lovejoy). 

Lovejoy Grid Coupling Misalignment Capacity 

(Standard and Spacer)

	Max Installation		Max Operational		Nominal
	Misalignment (in)		Misalignment (in)		Gap (in)
Size	Parallel	Angular (X-Y)	Parallel	Angular (X-Y)	+/-10%
1020	0.006	0.002	0.012	0.010	0.118
1030	0.006	0.003	0.012	0.011	0.118
1040	0.006	0.003	0.012	0.013	0.118
1050	0.008	0.004	0.016	0.015	0.118
1060	0.008	0.004	0.016	0.018	0.118
1070	0.008	0.005	0.016	0.020	0.118
1080	0.008	0.006	0.016	0.024	0.118
1090	0.001	0.007	0.016	0.028	0.118
1100	0.010	0.008	0.020	0.032	0.177
1110	0.010	0.009	0.020	0.035	0.177
1120	0.011	0.010	0.022	0.040	0.236
1130	0.011	0.012	0.022	0.047	0.236
1140	0.011	0.013	0.022	0.053	0.236

While the power density of a grid coupling is enviable, one final drawback to consider when selecting a grid (or gear) coupling is the fact that it requires lubrication (grease). Unlike elastomeric flexible couplings (which generally also provide system vibration dampening capability at lower power density levels), this means your maintenance team will need to periodically re-grease the coupling through the lubrication ports on the coupling's cover and be careful to avoid and/or properly address any grease leakage environmental concerns.

Alternatives to grid couplings include disc couplings (see Why to Switch from Grid to Disc Couplings), which avoid lubrication concerns but do not offer the grid coupling's level of vibration dampening, and jaw in-shear couplings (see Jaw In-Shear Couplings - A Straight Forward Value Add) which avoids lubrication but has a lower power density.

Universal Joints - Unique Solutions for Applications with Greater Misalignment

One of the primary functions of a flexible coupling is compensating for both angular and parallel misalignment.  But what if both the angular and/or parallel misalignment in the application is greater than what most flexible couplings can handle?  This is where a universal joint (u-joint) can provide a solution!  The unique features of a universal joint (and Lovejoy’s product offering) can provide customers with the best coupling fit for any application.

U-joints provide a unique function in the role of a flexible coupling.  Most flexible coupling designs offer from 1-1/2 to 6 degrees of angular misalignment.  The u-joint has the ability to compensate 25 degrees of angular misalignment as standard and up to 45 degrees of angular misalignment with modification.  The double u-joint type can provide larger amounts of parallel offset (amount dependent on size) than most other coupling types.  This provides customers with a flexible coupling solution that can accommodate large amounts of both angular and/or parallel misalignment.

The standard D type u-joint provides up to 25 degrees of angular misalignment.  The DD type allows for angular and larger amount of parallel offset (amount dependent on size) due to its double u-joint configuration.  The HD type allows for longer life due to the induction hardened yokes used in the assembly.  The NB style uses needle bearings to compensate for applications requiring higher speeds (up to 6000 RPM).  Finally, the LOJ and JR-4 types allow up to 45 degrees of angular misalignment in hand crank type applications. 

The amount of misalignment that a u-joint can compensate for is dependent on torque of the application, RPM, and the amount of angular misalignment in the application.  Should you have any further questions, consult your favorite technical support representative for proper sizing and support.  

Recommended Follow-On Reading: For a more complete perspective on general flexible power transmission couplings in general, excluding universal joints, please read: Flexible Coupling Basics - A Quick Primer

Author Credit: This article was written by Charlie Mudra (National Accounts Manager, Lovejoy, Inc.)

What is Static and Dynamic Alignment?

While we have previously identified that alignment is the primary cause of premature coupling failures (see post here), a further explanation of static versus dynamic alignment is in order.

Static alignment is the condition of the machinery at rest (think of the alignment we perform when the equipment is first installed).  Static alignment gives us the opportunity to correct issues such as soft foot, gross misalignment and to bring the system to within specifications.

Dynamic alignment is the condition of the machinery during sustained operation.  Think of an electric motor moving from its mechanical center to the electrical center, the thermal growth experienced by an internal combustion engine or a shaft moving axially in response to forces in the machine train.  It is common to perform a “hot alignment check” on equipment.  A hot alignment check is when the machinery is allowed to achieve its operational steady state condition (i.e. after a compressor train has operated for a minimum of 24 hours) and is then shut down with the express intention of quickly performing an alignment verification. 

Depending in the machinery, dynamic alignment is preferred as the equipment will experience measurable changes and can result in the equipment operating beyond alignment specifications from the initial static condition.

Remember to always align equipment to the tighter of either the coupling or equipment specifications!

Top Reason for a Coupling Failure

Coupling failure issues can be traced down to multiple types of failure modes. However, I have found that a few are the primary culprits for all types of couplings.

The #1 failure mode that causes the majority of the premature failures we see can be attributed to one aspect:

IMPROPER ALIGNMENT (angular, parallel or axial)

Some of the remaining common failure modes are (in no particular order):

• Lack of lubrication (Gear and Grid couplings)
• Improper torquing of fasteners
• Excessive torque
• Environmental
• Excessive vibration induced by mating machinery

While multiple issues can occur that will lead to a premature coupling failure, it is a good idea to start with the alignment of the machinery when determining a root cause. 

Used properly, dial indicators (lower cost & pictured above) and laser alignment tools (premium/higher cost) are both effective tools to consider when aligning a coupling.

Recommended Follow On Reading: For a deep dive into specific failure modes, inclusive of photographs, check out the following four articles below.

 

Coupling Failure Analysis - Jaw Couplings (includes hub & spider photos)

Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

Grid Coupling Failure Analysis (includes photos) 

Coupling Peak Torque Failure at Keyway