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Flexible or Rigid Coupling
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(Mechanical)
(OP)
6 Apr 07 03:25I am going to select a pump which is of following characteristics;
TYPE - Vertical
Fluid - Steam condensate
Capacity - 50 m3/hr
Temp - 98 deg C (max)
Seal - Mechanical type
Now my question is "Which type of coupling whould be better for this vertical condensate pump".
My experience is that, there are certain manufacturers which offer rigid coupling in case the pump thrust is faced by motor. But for that arrangement we have to align the pump using dowels on motor foots and its checking is tricky and quite labourous. Ofcourse there is no problem during operation.
Can a flexible coupling be a better choice for this arrangement or it can effect the performance of mechanical seal during long run.
Thanks in advance.
Replies continue below
(Mechanical)
6 Apr 07 05:07A lot will depend on the pump type / motor configuration and how much room /space is available to fit a coupling. You have given very limited detail so it not feasable to give you any difinitive answer.
(Mechanical)
(OP)
6 Apr 07 06:38Thanks for the reply Artisi.
At the time pump type and motor configurations are not final. The Situation is in requisition stage. If the configuration is known then it will be quite straight to finalize the coupling, for e.g if the vendor is giving the thrust bearing in motor then the whole of the pump load will be bear by the motor and in this case he will use rigid coupling. On the other hand, if the vendor is giving thrust bearing in pump and will not allow the motor to take up the load then he will use flexible coupling.
For now it is up to me to select the type of the configuration to be used.
Please guide me which configuration will be more effective and efficient from the maintenance and performance point of view.
Thanks.
(Mechanical)
6 Apr 07 07:28For me I would go with seperate bearings in pump and motor with a flexible coupling between. But up to you, if your specifying the unit at this stage you make the decisions on what you want.
(Mechanical)
6 Apr 07 11:09Quote:
For me I would go with seperate bearings in pump and motor with a flexible coupling between. But up to you, if your specifying the unit at this stage you make the decisions on what you want.
I second this statement. An engineer before me did a gear pump that was driven by a very powerful kollmorgen brushless DC motor with a splined rigid couple. I was his replacement and got to fix the design.......we tried a crown splined rigid couple that allowed for some misalignment but the ultimate solution was a flexible couple.
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(Mechanical)
6 Apr 07 14:52I would almost always prefer a flexible coupling. The only exception might be if I did not have enough head room for the extra height required with the flexible coupling option. The reason I prefer flexible is the distance from the impeller to the first true bearing. With a rigid coupling, the first true bearing is in the motor, quite a long distance from the impeller. Even with a support bushing, this long overhung shaft allows for more shaft deflection. This makes it very difficult to maintain tight wear ring clearances and plays havoc with the mechanical seal. Plus, those rigid couplings are very touchy. The tiniest little burr on the shaft can result in very high run-out out at the end of the shaft.
Johnny Pellin
(Mechanical)
(OP)
7 Apr 07 00:51Thanks for the replies guys.
I have also considered the same but a senior machinery engineer told me that you also have to see the fluid which you are pumping. Mechanical seals have very fine clearances between the seal faces and donot encourage the axial movement to be acted on them in vertical position. In my case i am going to pump steam condensate collected from a condenser and the temperature is almost 95 deg C. The axial play which the flexible coupling allows to encounter slight misalignment can cause the seal faces to open with time and can allow leakage.
Is the theory correct and flexible coupling can act in this way....
Although the designs with flexible coupling in vertical pumps are there and are proving successful, i mostly worked with rigid couplings at the fertilizer plant in condensate service and they are trouble free.
Please guide me as i have to select a trouble free design for the new installation.
(Mechanical)
7 Apr 07 05:37I can not believe that you could ever want the coupling to maintain the seal faces together, surely that is a function of the pump bearings. The coupling whether rigid or flexible should not be allowed to influence the seal.
(Mechanical)
7 Apr 07 09:45There should be no more axial movement allowed with a flexible coupling than there would be with a rigid coupling. In either case the shaft is restrained axially by ball bearings (motor bearings or pump bearings). In my experience motor manufacturers tend to leave more axial float (axial clearance) in their ball bearing arrangements than do pump manufacturers. If this is the case, then you will get less axial movement with the flexible coupling with the pump shaft supported on its own bearings. You describe this as a steam condensate pump. Most of the ones we have are vertical turbine pumps rather than vertical in-line. This affects some of my earlier comments. A vertical turbine pump will have a line shaft down to the impellers in the bottom of the "can" and thus will have support bushings even if it has the flexible coupling configuration. My concerns about radial movement (shaft deflection) are not valid in this case. The shaft will have the same opportunity for radial displacement regardless of the choice of coupling arrangements.
Johnny Pellin
(Mechanical)
(OP)
8 Apr 07 10:24Thanks guys.
The information provided is very helpful and surely help me out in my decision.
I would go for flexible coupling design. I have also come across with a design in which there is an intermediate bearing housing between pump and the motor. This design involves two set of coupling, one between motor and bearing housing which is flexible and the other between housing and pump which is rigid. In this arrangement we have a dual advantages of using flexible coupling for better support and rigid coupling for ease of maintenance i-e removing and installing mechanical seal.
(Mechanical)
9 Apr 07 16:22i agree with JJ. i have just gone through with vertical turbine pump requistion for wastewater application. we opted for flexible couplings and about bearings issue raised by ARTISI, in this case all bowl shaft, column section shaft and head shaft are supported by bushings (journal bearings)and in motor they provide thrust bearings which can handle all the down trust and up thrust(occasional). Make sure that impellers are hydraulically balance inorder to redice axial thrust on shaft which is critical in vertical application due to Shaft run out.
(Mechanical)
11 Apr 07 10:59One thing I'd like to add. With vertical turbine pumps that have mechanical seals I prefer a spacer coupling. Otherwise, the user probably will be required to remove the motor every time they service the mechanical seal. In some applications this can be quite often. Instead of the job being handled by a lone mechanic with a wrench or two they must also have an electician and sometimes a crane. I've seen installations that required a $/day crane to change out a $500 mechanical seal.
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(Mechanical)
11 Apr 07 13:23yep agree with Slickstir, we have dry spacer type flexible coupling.
(Mechanical)
19 Apr 07 08:45Ahhhh.
Design For Maintainability.
When talking with folks who have had to fix things it takes about one comment in a design review.
On the other hand, I witnessed a several day "discussion" with a stubborn apartment dwelling Russian engineer about providing a decent sized access hole for assembly and maintenance.
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Mixers are common industrial devices that consist of a motor and a propeller or paddle with a coupling acting as the connection point. Mixers are flexible, adaptable pieces of equipment found in a range of industries, from food processing to mining to chemical facilities.
Choosing the correct coupling for a mixer can be challenging as it needs to have an exact, rigid fit . This blog will highlight how to determine the right coupling for your mixer and how our configurator tool can help you configure your rigid coupling to your needs.
What Is a Rigid Coupling?
Unlike flexible couplings, a rigid coupling does not allow any radial or axial motion between the driving shaft and the driven unit. It is primarily used for vertical applications. Rigid coupling types include:
One-piece split couplings when there is access to both the motor shaft end and the paddle attachment.
Two-piece split couplings for easy assembly when in-line installation is needed.
Three-piece split couplings when the end of the shaft will remain fixed while the other is moved or changed (very common for mixer situations when paddles would be changed or replaced).
Flanged couplings when mounting options are needed.
On mixers, the motor is bearing-supported, but the shaft on the impeller side is not. Using a flexible coupling would require close bearing support on the paddle, but the sleeve-like design of rigid couplings eliminates the need for a bearing to support the paddle. This makes rigid couplings the only viable option for mixers.
Rigid couplings have a simple and durable design, allowing greater transmission of power to mixers. They are more cost-effective than other options and offer more design flexibility. While requiring exact alignment between shafts, rigid couplings offer a stronger connection, higher torque, and more precision than flexible couplings.
Advantages of Rigid Couplings for Mixers
Rigid couplings provide advantages, such as:
: They can transfer torque efficiently between connected shafts.
: Manufacturers such as Stafford Manufacturing can produce custom and standard rigid couplings at affordable prices.
Its high torsional stiffness enables better positioning.
: Rigid couplings offer high precision with almost no windup or backlash.
: They can establish shaft alignment between the connected parts and the motor.
: Rigid couplings have a simple design, allowing for easy assembly, disassembly, and maintenance operations throughout the coupling&#;s lifespan.
Considerations for Selecting a Rigid Coupling for a Mixer
Choosing the best coupling for a mixer requires precision. Stafford Manufacturing&#;s configurator tool can help you customize a rigid coupling that suits your exact needs. Configure it by bore diameter, coupling style, coupling material, screw material, finish, and other options available upon request.
Here are more details about the top considerations for choosing a rigid coupling for a mixer.
Bore Diameter
Bore diameter ranges from ¼&#; to 4&#;. Choosing the right diameter is key to ensuring a precise alignment.
Coupling Style
Coupling styles include one-piece, two-piece, and three-piece. One-piece styles have high torque capacities, unlike set screw type couplings. Two-piece styles allow for easy assembly and adjustment. Three-piece styles are used to maintain one shaft&#;s positioning while the other is adjusted.
Material Selection
Material selection is based on the substance your coupling is exposed to. Coupling materials include brass, 303 and 304 stainless steel, weldable steel, carbon steel, and aluminum.
How to Choose the Best Coupling for a Mixer
Consider these factors when deciding on the best coupling for your mixer:
Shaft size
Keyway size if needed
Required torque capacity
Bore diameter
Material requirements for specialized (i.e. food-grade) applications
Stafford&#;s rigid couplings meet these criteria as they are designed with end users&#; needs in mind. Our rigid couplings come in different types, from the common one-piece split clamp coupling to precision sleeve couplings.
Choose Stafford as Your Coupling Manufacturer
Stafford is the leading manufacturer of high-quality shaft couplings and rigid couplings. With the help of our configurator tool, you can determine the right rigid coupling for your needs. Contact us to learn more or request a quote for our products.
If you are looking for more details, kindly visit Flexible Coupling Types.