Universal Joints vs Other Coupling Types

Universal joints (often called u-joints, Cardan joints dates back as far as 1545, when Italian mathematician Gerolamo Cardano first suggested using a universal joint for power transmission purposes. Today universal joints are found in countless applications and industries.

Given universal joints impressive ability to handle angular misalignment (generally up to 25 degrees)... and their near ubiquitous use in some applications... some might wonder why other coupling solutions exist at all.

What exactly makes universal joints so "universal", and where do universal joints fall short?

Fantastic for Misalignment Handling

In general, shaft misalignment does not do good things for the life expectancy of any flexible coupling or joint (be it a universal joint or other designs), but, in many applications, large angular misalignment handling is unavoidable. For such applications, universal joints are often ideal.

Universal joints can also handle significant parallel misalignment needs by making use of two joints in series (as shown in the DD and DDX types at right).

Lastly, while not a standard feature, axial misalignment can also be handled with universal joints by adding an additional sliding shaft or spline feature to the joint assembly.

Primary Drawbacks of Universal Joints

#1. Space (Long Footprint)

Relative to other coupling solutions, universal joints do not have the most power dense design (i.e. - gear couplings), in particular, in regards to overall length/required distance between shaft ends. Assuming parallel misalignment must also be accommodated (in addition to angular), two joints in series must be used. Two joints are also needed if near constant velocity is required (see drawback #2 below).

 #2. Speed Fluctuation

While many coupling types inherently provide constant velocity output (i.e. - disc couplings), a single universal joint, subject to angular misalignment, will not provide a variable output velocity. This velocity variation gets worse (larger) as the angular misalignment that joint sees... which can be a source of torsional vibration into a given power transmission system.

This drawback was first noticed and recorded by Robert Hooke between 1667 and 1675. Fortunately, Hook realized that the output fluctuation can be largely eliminated (turned into to a near constant velocity joint) by using 2 standard joints back to back (i.e. - DD and DDX type) where the center member's yoke ears are aligned (the two joints are 90 degrees out of phase) and joint angles are equal. This is because the second joint (being 90 degrees out of phase with the same angle) compensates/cancels out the velocity changes from the first joint. Such joints are often referred to as Double Cardan.

Note: In addition to the Double Cardan, where constant velocity is mission critical, there are several other unique constant velocity joints (often called cv-joints) available in the market. These designs tend to be significantly more complex/expensive, and may have a reduced load carrying capacity. 

#3. No Dampening

Universal joints are generally not designed to dampen vibrations, inclusive of any torsional vibrations a given universal joint might introduce into a system (from output velocity fluctuation). 

Depending on the application, the inability of a joint to dampen vibration may be viewed as a major weakness relative to other couplings designs. Many other designs are either inherently good at dampening vibrations (i.e. - elastomeric coupling types to include Jaw, S-Flex & metallic Grid couplings) and others are designed explicitly to dampen specific vibrations relative to a specific user application  (i.e. - torsional couplings).  

#4. Maintenance 

Unlike many elastomeric and some metallic coupling designs, universal joints generally include lubricated metal on metal connections. As such, universal joints generally cannot be viewed as "maintenance-free". (Periodic inspection/re-lubrication is required, and the joint will still eventually wear out.) Indeed, a number of companies exist and thrive by servicing and rebuilding large high torque industrial universal joints.

 

 

Big Picture

Big picture, universal joints are a fantastic solution for the right applications: applications where angular misalignment (or parallel misalignment) is unavoidable, where space permits, where constant velocity is not critical (or can be overcome with a Double Cardan), where a lack of dampening is not a concern, and where some maintenance is acceptable.

Within the category of universal joints there is a wide breadth of product options. They include the following four subgroups.

Pin & Block designs (which encompass the majority of Lovejoy's product lineup - show above) are the fundamental "bread and butter" design. They are well-suited to work in the lower speed ranges (up to 1,750 rpm), and are able to transmit a fairly significant amount of torque relative to the outside diameter. These designs include a center block and two pins connecting the yokes to the block, and are readily available in several of grades of steel (inclusive of stainless). Lubrication and use of rubber boots around the joint is required for optimal/extended life of these designs.

Needle Bearing designs (also available by Lovejoy) operate at increased speeds (up to 6,000 rpm) and generally have reduced backlash, but are rated to carry a bit less torque (relative to Pin & Block). Needle Bearing universal joints generally come per-lubricated and sealed. Like Pin & Block, boots & lubrication are still required for optimal wear/system life.

Ball & Socket universal joints are another type (not offered by Lovejoy) that generally eliminate backlash, but at the expense of torque and speed.

Cross & Bearing universal joints replace the block and pins with a hardy single center "cross" member. These designs (not currently offered by Lovejoy) are generally seen on higher torque applications inclusive of large mobile equipment and industrial drive shafts.

If you are confident a universal joint is what you need for your given application... Lovejoy's universal joint product catalog is readily available for download, and your closest Lovejoy distributor can be found here.

Should you have any further questions or concerns regarding universal joints, please visiting Lovejoy's universal joint product page or contacting a Lovejoy product specialist.

And if you would like to explore other coupling solutions (outside of universal joints), please consider starting by reviewing Lovejoy's Coupling Preselection Guide and/or reading The Coupling Handbook. 

Keeping the Wastewater Industry Pumping - The Right Coupling Design Can Make A Huge Difference

Rigid adjustable coupling sized for a vertical pump

In the previous blog post we talked about the importance of pump impeller clearances -- where having the “just right” clearance significantly influences the efficiency of the pump. In this "Part 2" post, we will dive into how Lovejoy has developed and optimized their coupling designs to address the needs of the industry.

The design of Lovejoy's Rigid Adjustable Type Couplings inherently fit better into a vertical pump application because all of the adjusting nuts and bolts are readily available and exposed, to help the operator replace components without taking apart the machinery.  (Other coupling styles are encapsulated and must be removed entirely from the pump in order to provide routine maintenance.)

The use of this coupling helps eliminate the care that must be taken when attaching piping to the discharge of vertically mounted pumps to avoid piping stress (axial or radial force) on the discharge port.  Such force will push the pump out of vertical alignment, causing premature failure.  No piping stress can be allowed against the discharge connection. The coupling’s rigid hubs are manufactured with clearance-fit bores and keyways which, allow easy installation on the either motor or pump shafts. Twelve sizes cover bore requirements ranging from 0.44" to 10.5" with square key (or to 276mm with metric key), horsepower-per-100-RPM ratings up to 2,164, and thrust capacities up to 400,000 lbs.

Impeller shaft mounted in flow tube, waiting for installation

The upper (motor) hub is installed to the motor shaft by axial keyway and hangs on a split thrust ring fitted into the shaft's circular keyway.  The lower (pump) hub is installed to the pump shaft by axial keyway, hangs from an adjusting nut turned onto the pump shaft's threaded end. The adjusting nut has a smooth cylindrical shape matching the hubs' outside diameter, with radial holes to allow turning with a spanner wrench.   After adjustment is set, hubs and adjusting nut are locked together with axial through-bolts.  The Type IV spacer engages the lower hub's through-bolts on one end and bolt separately to the upper hub on the other end. With the ease of installation and removal of the coupling the mechanical seals, which have an 85% or higher failure rate, can be serviced and corrected, helping avoid unnecessary down time and excessive operating expense.  Seals should run until the sacrificial carbon face is worn away, but in more than 85% of the cases the seal fails before this happens.

Operators should know where the best efficiency point (B.E.P.) is on a particular pump, and how far it is safe to operate off the B.E.P. with a mechanical seal installed.  Washing down the pump area with a water hose would cause premature bearing failure when the water penetrates the bearing case. The pumped product will affect the life of the mechanical seal and environmental controls are necessary.  If you are not using cartridge seals, adjusting the open impeller for efficiency will shorten the seal life.  In most cases the seal will open as the impeller is being adjusted to the volute. In addition, cycling pumps for test will often cause a mechanical seal failure unless an environmental control has been installed to prevent the failure. 

Mechanical seals should be positioned after the impeller has been adjusted for thermal growth. This is important on any pump that is operated above 200°F (100°C) or you will experience premature seal failure. Some elastomeric seals will be affected by steaming the system.  A great deal of caution must be exercised if a flushing fluid such as caustic is going to be circulated through the lines or used to clean a tank.  Both the elastomer and some seal faces (reaction bonded silicon carbide is a good example) can be damaged.  If the elastomer is attacked, the failure usually occurs within one week of the cleaning procedure.  The stuffing box must be vented on all vertical centrifugal pumps or otherwise air will be trapped at the seal faces that can cause premature failure of many seal designs.  That is why the right seal design and ease of maintenance of the coupling can make such a huge difference in the efficiency of an operation like a wastewater treatment plant.

The RA/RAHS Series offers both close-coupled, Type II, and spacer styles, Type IV, in standard (RA) type and high-speed (RAHS) type, which is specially balanced to API 610 tolerances.  Either style allows the user to adjust the impeller in order to maintain the appropriate gap between the impeller and housing to maintain the appropriate gap between the impeller and housing to maintain the correct low rate. The spacer styles provide a center section that can be removed to allow the impeller to lift out for maintenance without disturbing and having to re-set its vertical clearance adjustment, or dismounting either the motor or pump housing. All styles and types are available in either steel or stainless steel, for broad applications across water, wastewater, power generation, refinery, and many other industries. 

Choosing the right coupling is just one step, but a very important one, in the process of keeping America's wastewater treatment plants pumping at peak efficiency.

About the Author: Kevin Remack, Vice President of Sales for Lovejoy Inc., has many years of experience in the mechanical power transmission industry and has been a longtime champion of Lovejoy's engineered coupling solutions.

The Importance of Wastewater Pump Impeller Clearances

Based on population estimates for 2016, The United States Environmental Protection Agency (USEPA) projects that more than 18,000 wastewater treatment facilities will be needed to process over 32 billion gallons of waste per day.  Maintaining these facilities represent a huge challenge for operators in order to keep them running smoothly and continuously.

At the heart of a consistent and reliable operation of these facilities is the pump.  The pumps utilized in this industry are usually vertical such as, vertical turbine pumps, irrigation pumps, barrel pumps, fire pumps, or propeller pumps. These vertical pumps use a torsional, disc or gear coupling to stay in operation.

Vertical pumps typically are for medium head applications where the specific speed of the pump ranges from 4000 to 9000.  In a vertical pump, the pump impeller "sits" in a casing or bowl.  The outer diameter contours of the impeller vanes match those of the bowl so that the tips of the impeller vanes are always parallel to the surface of the bowl.  The parallel gap between the impeller vanes and the bowl, that is, the clearance, significantly influences the efficiency of the pump.

If the clearance is too large, water can re-circulate from the high-pressure portion downstream of the impeller (above the impeller) to the low-pressure portion upstream of the impeller (below the impeller). This not only causes a loss in efficiency of the pump, but it can also lead to accelerated erosion of the bowl.

On the other hand, if the clearance is too close, the surface hydraulic boundary layers of the impeller and bowl may interfere with each other. This causes the hydraulic friction due to viscous shear between the two boundary layers to increase, which decreases pump efficiency. Furthermore, if the clearance is much too close, the impeller and bowl may directly interfere and scrape on each other. This causes a significant decrease in pump efficiency. Energy intended for pumping water is diverted and consumed by the impeller grinding itself into the pump bowl. This contact causes permanent damage to both the impeller and bowl and shortens the service life of the pump.

Between the two extremes of too tight and too loose, there is a "just right" clearance dimension. This "just right" clearance dimension allows the boundary layers of the pump impeller and bowl to slide over each other with minimal shear, but is not so large as to allow excessive re-circulation between the upstream and downstream sides of the impeller. At the "just right" clearance, pump efficiency will be at maximum.  The manufacturer usually specifies impeller clearances.

All pumps will need adjustment during the course of operation to maintain the clearance dimension that ensures maximum pump efficiency. Any adjustment is accomplished by actually lifting the impeller upwards such that its measured vertical gap between the impeller and bowl is within the range of acceptable values provided by the manufacturer or by the engineer in charge.

Since the pump impeller and bowl themselves are normally immersed in water and are inaccessible, this is done at the top of the pump by loosening the pump shaft from the motor shaft at their coupling and allowing the impeller and shaft to rest on the bowl. This is the "zero" clearance position. The shaft is then lifted upwards, usually by tightening coupling bolts or adjustment plates, from the zero clearance position. The amount of lift is the amount of upward displacement of the shaft and impeller created by tightening the coupling bolts or adjustment plates from the zero position. The clearance measurement is usually made with feeler gages or dial indicators.

At the component level, this is where the right style of coupling can save the operator valuable time and energy, by offering the right combination of design, consistency and reliability. Very few manufacturers, such as Lovejoy Inc., provide Rigid Adjustable Type Couplings (like HercuFlex™). These couplings allow easy axial adjustment of impeller clearance in vertical pump installations designed for stationary applications, such as sewage digesters and lift stations, manure tanks, chemical sumps, and dry docks.

In conclusion, couplings from other manufacturers are encapsulated, and must be removed entirely from the pump in order to provide routine maintenance. In contrast, the design of the HercuFlex™ rigid adjustable coupling inherently fits better into a vertical pump application. All of the adjusting nuts and bolts are readily available and exposed, to help the operator replace components without taking apart the machinery – making routine maintenance a breeze when compare to other couplings.

For your reference, the coupling utilized in this application is the new generation of gear couplings from Lovejoy, HercuFlex™.  This revolutionized coupling offers increased nominal torque, larger maximum bore size and longer service life. Despite the advanced nature of these improvements, this Gear Coupling still utilizes the standard AGMA flange interface to ensure field interchangeability. Its RA/RAHS Series offers both close-coupled, Type II, and spacer styles, Type IV, in standard (RA) type and high-speed (RAHS) type, which is specially balanced to API 610 8th edition tolerances.


About the Author: Kevin Remack, Vice President of Sales for Lovejoy Inc., has many years of experience in the mechanical power transmission industry and has been a longtime supporter and champion of Lovejoy coupling solutions.