Jaw coupling spider materials hold the key to solving customer application challenges

Customers are confronted with a variety of different challenges regarding selecting jaw couplings inserts.  With four different insert materials to choose from, several details about the application should be supplied to offer the best coupling fit.  The amount of existing misalignment is a concern when selecting an insert.  Ambient temperature of the area where the coupling will operate is an important part of insert selection.  The amount of torque a particular spider can handle is also a very important factor in selecting the insert material.  By looking at the different inserts available, Lovejoy can evaluate the advantages and disadvantages of each material and select the best fit for the application.

The standard SOX spider or Buna-N material rubber offers the advantage of vibration damping.  The Buna-N also has good resistance to oil and greater misalignment capabilities than Hytrel and Bronze inserts.  Some of the draw backs to Buna-N are its lack of resistance to certain types of chemicals and having the lowest torque rating of all of the inserts.   

Urethane provides customers with better chemical resistance than the Buna-N material and 1.5 times the torque capacity of the Buna-N material.  However, the Urethane material does not dampen vibration as well as the rubber material.   

The Hytrel insert has the advantage of carrying the most torque of the elastomers.  Hytrel also has excellent resistance to many different chemicals and a temperature rating (250^o F) higher than the Buna-N and Urethane inserts.  However, due to the hardness of the material, Hytrel only offers about half the misalignment of the rubber insert and does dampen vibration.   

The Bronze material insert offers the highest temperature rating (450^o F) and has good chemical resistance characteristics.  Bronze also has a high torque rating similar to the Hytrel material.  The main disadvantage of a bronze insert is that it can only be used in applications running at 250 RPM or lower.   

With all of the these choices available, finding out application’s horsepower, speed, type of machinery to be connected, possible chemical exposure, and ambient temperature of coupling’s environment are critical.

By outlining the strengths and weaknesses of each type of spider material, it is clear that one spider does not fit all.  If this were true, there would be only one spider material.  By considering the application information, the best material choice can be made.  The result is the best fit for the application, which results in longer life for the insert. 

Recommended Follow-On Reading: For a more complete perspective on jaw couplings in general, inclusive of a second review of elastomeric spider options, please read: Jaw Coupling Overview - Features & Benefits, Design Basics, and Element Options

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

Bellows Couplings – Flexibility and Accuracy for Motion Control Applications

The category of motion control covers a wide range of application types.  Conveyors, machine tools, electronic monitoring devices, pick & place robots, and xyz gantry tables are just a few examples of the variety of these applications.  Motion control couplings need to be selected by evaluating which characteristics are most important to the proper function of the application.  Some applications can call for vibration dampening characteristics, misalignment compensation, and greater torque capacity or any combination of all three.  However, sometimes the accuracy or precision of an application is rated the most important characteristic.  An example of this would be a XYZ gantry table (i.e. CMM machine), which relies on accuracy to check critical dimensions of a particular product.  For these types of applications, the Bellows style coupling provides a compact coupling solution with a high amount of torsional stiffness, which translates into high accuracy for the application

Lovejoy Inc. offers two different size ranges in the Bellows coupling product line: the BWC and BWLC series.  The BWC coupling offers bore ranges of 1/8” to 3/4” and torque ratings of 3.5 to 89 in-lbs.  The BWLC coupling offers bore ranges 1/4” to over 1-1/2” and torque ratings of 159 to 1,328 in-lbs.  Both of these designs offer higher torsional stiffness than beam coupling designs and still retain zero backlash.  This combination is ideal for precision soldering equipment, pick & place machines, and measurement equipment.  The wide range of bore sizes and torque capacities allow for flexibility in selecting the right size coupling for the application.

Recommended Follow-On Reading: To gain a broader understanding of the landscape of Flexible Couplings, and where bellow motion control couplings fit in, we recommend you read the following article: Flexible Coupling Basics - A Quick Primer

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

Rolled Key Example in a Coupling Hub

One issue that we see from time to time is the when a key has rolled within the coupling hub/shaft keyway & key joint.  While the joint might not experience any external issues (such as a crack) the joint does need immediate attention.  

It is just a matter of time before the joint will experience an issue as the joint may no longer able to transmit the full torque capacity.  The upper right hand corner of the image above is a good example of a rolled key, with a visible gap seen between the shaft and coupling near the keyway. 

The coupling pictured above was taken out of service and removed to allow for a more complete inspection & further determination as to what steps were required to bring the coupling hub/shaft keyway joint up to acceptable condition.

Recommended Follow-On Reading: To learn more about the importance of inspecting shafts, shaft keyways, and key prior to coupling installation, we recommend the following two articles: Shaft Inspection Prior to Coupling Installation and Shaft Keyway Damage.

Shaft Inspection Prior to Coupling Installation

Prior to installing a coupling (new installation or replacing an existing unit) it is important to inspect the condition of the shaft.  If the shaft is sufficiently gouged, marred or otherwise distorted it may need to be dressed (reworked) prior to installing the coupling.  During laboratory testing of the coupling to shaft interface we found that a deformed shaft can reduce the torque capacity of the joint.  It is important to understand that the joint will only be as strong as the weakest link in the system.

An example of a shaft that needs to be dressed prior to coupling installation is shown above.

Recommended Follow On Reading: In addition to inspecting the shaft for damage prior to coupling installation, it is also important to inspect the shaft keyway and key for damage as well. The following is an article on shaft keyway damage: Shaft Keyway Damage

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!

Maintenance Procedures for Couplings

I recently gave a presentation on couplings and one of my fellow presenters had an interesting story.

A consultant was hired by a refinery operation in South America to study why the MTBF (mean time between failures) was significantly different than their peer group.  Upon investigate this was discovered:

Needless to say the maintenance procedures & operations was given a detailed review….

It is import to understand what the root cause of an issue is and take to time to correct the situation. While we are all pressed for time, would you rather have it done it right the first time or find the time to do it all over again?

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

Sizing a Coupling – Pay Attention to the Driver!

One of the steps in properly sizing a coupling is to understand the application.  Continuous operation vs stop-start, high inertia loading, excessive vibration, etc…  The severity of the application is used to modify the calculated application torque to truly represent the usage of the system.  A handy reference guide is the Driven Machine Service Factor charts that coupling manufacturers publish.  This will typically give a number between 1.0 and 3.0. 

An item that is often overlooked but should be considered for specialized applications (infrequently, but particularly pertaining to gear and disc couplings) is the driver.  When a Variable Speed Electric Motor or Internal Combustion Engine is used as the driver within a coupled system, the torque variations experienced by the coupling are more severe than with a standard electric motor system. This variation is included in the formula:

Application Torque x [Service Factor (Driven) + Service Factor (Driver)] = Selection Torque

The Driver Service Factor can vary from 0.0 (standard electric motor) to 1.5 (low cylinder count internal combustion engines).

As you can see, in specific specialized applications, the Driver can potential have a greater impact on sizing the coupling than the Driven unit!

Should I rotate shaft keyways 180° apart when installing a coupling?

A common question that I am asked is if the keyway on the drive and the keyway on the driver shafts should be oriented 180° apart.

My response is twofold. If the unit is designed and manufactured as a single system, the orientation of the keyways should not matter as the key length will correspond to the bore length of the coupling (and the keyway cut into the shaft should also be only just long enough to properly fit the coupling). 

However, some applications have a longer key & keyway than what is needed for the coupling.  The longer key may cause an imbalance in the system if both keyways in the shafts are aligned. It is with this potential imbalance in mind that the “rule of thumb” to orient the keyways 180° apart was created.

Big picture, if you want to minimize imbalance in a system when installing a coupling the 180° keyway "rule of thumb" is certainly a worthwhile. While it may not always be required, it is a good "best practice" to follow.