How to Choose servo motor joint?

Choosing the most appropriate type of coupling to use in servo applications can be confusing. This article examines the pros and cons of the various technologies.

ARCSEC DRIVE are exported all over the world and different industries with quality first. Our belief is to provide our customers with more and better high value-added products. Let's create a better future together.

Selecting a coupling for a servo application can be a complex process. It involves many different performance factors, including: torque, shaft misalignment, stiffness, rpm, space requirements, and others, that all must be satisfied for the coupling to work properly. Before selecting a coupling, it is helpful to know the specifics of these issues for the application for which the coupling is to be used. Many different types of servo couplings exist with their own individual strong and weak points. This article is designed to introduce end users to the different types of couplings available for servo applications. It also helps the user select the proper coupling for their application by highlighting the factors that should be considered in the decision making process and how they relate to the different product offerings available.

Check out our Servo Coupling Comparison Guide for a performance breakdown of every Ruland servo coupling.

Beam Couplings

Beam couplings are manufactured from a single piece of material, usually aluminum, and utilize a system of spiral cuts to accommodate misalignment and transmit torque. They generally have good performance characteristics and are an economical choice. For many applications, beam couplings are a good place to start. The single piece design allows the coupling to transmit torque with zero backlash and no maintenance required.

Two basic variations on this theme exist: a single beam style and a multiple beam style. The single beam style has one long continuous cut that usually consists of multiple complete rotations. This results in a coupling that is very flexible and yields light bearing loads. It is able to accommodate all types of misalignment, but works best with angular misalignment and axial motion. Parallel misalignment capabilities are reduced because the single beam is required to bend in two different directions at the same time, creating larger stresses in the coupling that could cause premature failure.

Although the long single beam allows the coupling to bend easily under misalignment conditions, it has the same affect on the rigidity of the coupling under torsional loads. The relatively large amount of windup under torsional loads adversely affects the accuracy of the coupling and reduces its overall performance.

Single beam couplings are an economical option that are best utilized in lower torque applications, especially in connections to encoders and other light instrumentation.
Multiple beam couplings, which usually consist of 2 or 3 overlapping beams, attack the problem of low torsional rigidity.

The use of multiple beams allows for the beams to be shorter without sacrificing much of the misalignment capabilities. The shorter beams make the coupling stiffer torsionally and overlapping them so the beams work in parallel increases the allowable maximum torque. This makes them suitable for use in light duty applications with connections such as a servo to a leadscrew. This increase in performance does not come without penalty: bearing loads are increased by a sizeable amount over the single beam variety but, in most cases, remain low enough to protect bearings effectively. Some manufacturers take the multiple beam concept to another level, also. Instead a single set of multiple cuts,  two sets of multiple cuts are utilized. The use of multiple sets of cuts gives the coupling additional flexibility and misalignment capability.

It also adds a dimension to the misalignment capability of this type of coupling by more readily accepting parallel misalignment. In constrast to couplings with one beam or a single set of beams, under parallel misalignment, one set of beams bends in one direction and the second set bends in the other direction making the coupling more adaptable to this type of misalignment.

Most commonly, aluminum versions of these couplings are used. However, several manufacturers offer designs available in stainless steel also. The use of stainless steel, in addition to corrosion protection, also increases the torque capacity and stiffness of the coupling to sometimes double that of aluminum parts of the same design. The increase in torque and stiffness is off-set by a dramatic increase in mass and inertia.

Often times the negative affects will outweigh the positives and force the user to look for another type of coupling. In applications using smaller motors, a large percentage of the motor’s torque is used to overcome the inertia of the coupling, seriously reducing the performance of the system.

Oldham Couplings

The oldham coupling is a three piece coupling comprised of two hubs and a center member . The center disk, which is usually made of a plastic or, less commonly, a metallic material, is the torque transmitting element. Torque transmission is accomplished by mating slots in the center disk, located on opposite sides of the disk and oriented 90 degrees apart, with drive tenons on the hubs. The slots of the disk fit on the tenons of the hub with a slight press fit. This press fit allows the coupling to operate with zero backlash.

Over time it should be noted that the sliding of the disk over the tenons will create wear to the point the coupling will cease to be zero backlash.The disks, however, are inexpensive items that are easily replaced and a new insert will restore the couplings original performance

In operation, the center element slides on the tenon of the hub to accommodate misalignment. Because the only resistance to misalignment is the frictional force between the hub and disk, oldham couplings have bearing loads that do not increase as misalignment increases. Unlike other types of couplings, there are not any bending members which act as springs, causing bearing loads to increase as the shafts become further misaligned.

However, these ratings can be surpassed at the expense of coupling life. The ability to choose different disk materials is an advantage of this type of coupling. Several manufacturers offer choices of material to meet application needs.  Generally. one material is best used zero backlash,  high torsional stiffness and torque are required, and another material for applications that have less precise positioning requirements, do not require zero backlash, and can benefit from a coupling that can absorb some vibration and reduce noise. Nonmetallic inserts are also electrically isolating and can act as a mechanical fuse. hen the plastic insert fails, it breaks cleanly and does not allow any transmission of power, preventing other damage from occurring to more expensive machinery components. The area this design is particularly well suited is handling relatively large amounts of parallel misalignment (from .025" to .100" or more depending on coupling size). Coupling manufacturers generally provide smaller misalignment ratings that allow users to obtain maximum life.

Zero Backlash Jaw Couplings

There are two general types of jaw couplings: the conventional straight jaw couplings and curved jaw zero backlash jaw couplings. Conventional straight jaw couplings are not typically well suited to servo applications accuracy of torque transmission is required.

Zero backlash jaw couplings are a variation on the same theme, but the differences in design make them well suited to servo applications.The curved jaws help to reduce deformation of the spider, limiting the effects of centrifugal forces during high speed operation.

Zero backlash jaw couplings consist of two metallic hubs and an elastomer insert , which is commonly referred to in the industry as a “spider".The spider is a multiple lobed insert that fits between the drive jaws on the coupling hubs with a jaw from each hub fitted alternately between the lobes of the spider. As in the oldham coupling, there is a press fit between the jaws and the spider that allow the coupling to remain zero backlash. In contrast to the oldham coupling, the torque disk is in shear under torsional loads, the jaw coupling’s spider operates in compression.

When using a zero backlash jaw coupling the user must be careful not to exceed the manufacturer’s rating for maximum torque with zero backlash which can be significantly below the physical limitations of the spider. If this occurs, the spider can be compressed so that there is no longer a preload and backlash will occur,  possibly without the user noticing until a problem occurs.

Jaw couplings are well balanced and are able to handle high RPM applications very well ( manufacturers rate speeds up to 40,000 RPM ), but are not able to handle very large amounts of misalignment, especially axial motion. Large amounts of parallel and angular misalignment cause bearing loads that are higher than most other types of servo couplings. Another factor that the user must be aware of is the situation when a jaw coupling fails. If a spider fails, the coupling will not disengage. The jaws from the two hubs will mate similar to teeth on two gears and continue to transmit torque with metal to metal contact which, depending on the application, may be desirable, or could cause problems in the overall system the coupling is installed. An advantage of the jaw coupling is the ability to mix and match spiders based on the application. Manufacturers of zero backlash jaw couplings offer multiple materials with different hardnesses and temperature capabilities that allow the user to choose exactly the insert that meets the application’s performance criteria.

Disc Couplings

Disc couplings are comprised of, at a minimum, two hubs and a thin metallic or composite disc that is the torque transmitting element. The disc is fastened to the hubs usually with a tight fitting pin that does not allow any play or backlash between the disc and hubs. Some manufacturers offer disc couplings with two discs separated by a rigid center member and attached to a hub at each end.

The difference between the two variations is quite similar to the difference between the single beam style coupling and the multiple beam coupling consisting of two sets of cuts.The single disc coupling is not very adept at accommodating parallel misalignment due to the complex bending of the disc that would be required.The two disc style allows each disc to bend in opposite directions to harness the parallel off-set.

The properties of this type of coupling are similar to that of bellows couplings. In fact the way the couplings transmit torque in general is very similar. The discs are very thin, allowing them to bend easily under misalignment loading which allows the coupling to accept large amounts of misalignment (up to 5 degrees) with some of the lowest bearing loads available in a servo coupling. Torsionally, the discs are very stiff. The disc coupling has stiffness ratings slightly lower than that of bellows couplings. A downside to these couplings is that they are very delicate and prone to damage if misused or installed improperly. Special care must be taken to insure that the misalignment is within the ratings of the coupling for proper operation.

Bellows Couplings

The bellows coupling is an assembly of two hubs and a thin walled metallic bellows. The assembly is created in most cases by either welding the hubs to the bellows or by using an adhesive of some variety. Although other materials can be and are used, the two most common materials for the bellows are stainless steel and nickel.

Nickel bellows are manufactured using an electrodeposition method. This method involves machining a solid mandrel in the shape of the finished bellows. The nickel is electrodeposited onto the mandrel and the mandrel is then chemically dissolved, leaving behind the finished bellows. This method allows the manufacturer to precisely control the wall thickness of the bellows and also allows for thinner walls than other methods of bellows forming. The thinner walls give the coupling greater sensitively and responsiveness making them ideally suited for extremely precise small instrumentation applications. However, the thinner wall also reduce the torque capacity of the bellows putting a limiton useful applications.

Stainless steel bellows are stronger than nickel versions and are usually manufactured with a process called hydroforming. A thin walled tube is placed into a machine and hydraulic pressure is used to form the convolutions of the bellows around specialized tooling. The characteristics of bellows make them an ideal method for transmitting torque in motion control applications. The uniform thin walls of the bellows allow it to bend easily under loads caused by the three basic types of misalignment between shafts (angular, parallel, axial motion). Generally bellows allow for up to 1-2 degrees of angular misalignment and .010" - .020" of parallel misalignment and axial motion.

The thin, uniform walls result in low bearing loads that remain constant at all points of rotation, without the damaging cyclical high and low loading points found in some other types of couplings. All of this is accomplished while remaining rigid under torsional loads. Torsional rigidity is a key factor in determining the accuracy of the coupling. The stiffer the coupling, the more accurately motion is translated from the motor to the driven component. In the area of servo couplings, bellows type couplings are some of the stiffest available, making them ideal in high performance applications that require a high degree of accuracy and repeatability. Some manufacturers offer bellows couplings with stainless steel hubs, which can be useful in applications corrosion resistance is important. The mass of stainless steel hubs does reduce some of the benefit of this type of coupling. The use of aluminum hubs with a bellows results in a coupling with very low inertia, a feature that is very important in today’s highly responsive systems. Some manufacturers of bellows coupling balance their couplings as a standard offering making them well suited for higher rpm applications (10,000 RPM ) as well.

Rigid Couplings

As the name implies, rigid couplings are torsionally rigid couplings with virtually zero windup under torque loads, but they are also rigid under loads caused by misalignment. If any misalignment is present in the system the forces will cause the shafts, bearings or coupling to fail prematurely. This also means that the couplings cannot be run at extremely high rpm’s since they cannot compensate for any thermal changes in the shafts that can be caused by heat buildup from high speed use. This also means that the couplings cannot be run at extremely high rpm’s since they cannot compensate for any thermal changes in the shafts that can be caused by heat buildup from high speed use. However, in situations misalignment can be tightly controlled rigid couplings offer excellent performance characteristics in servo applications.

Although in the past many people wouldn’t consider using this type of coupling in a servo application, recently smaller sized rigid couplings, especially in aluminum, are increasingly being used in motion control applications due to their high torque capacity, stiffness, and zero backlash.

In conclusion

Choosing the proper servo coupling for an application is a critical part of total system design and greatly affects its overall performance capabilities. For this reason, considering the coupling early in the design process and aligning the coupling performance attributes with the functionality goals of the system can eliminate many problems that typically occur in motion control applications. Each of the couplings discussed have their own individual characteristics that make them ideal for many different uses. A single type of coupling, however, cannot be applied to every application in the field. This leads to the wide variety of couplings currently available and gives the design engineer the ability to select the best possible coupling to maximize system performance and durability

If you want to learn more, please visit our website servo motor joint.

hi guys,

i'm a beginner in robotic. only knw a bit about coding (programming).

anyway, i'm starting my project today, 6 arm robot with a gripper, 7 servo actually as i need 1 small servo to control the gripper to hold the load. the length of the arm would be 70cm. i'm expecting to be longer like 150cm total with payload 1kg.

here is my design in attachment. sorry it look like back to 19th century but yes, i'm a programmer, not a cad and even i don't have any idea about autocad and 3ds max. perhaps i should giveit a try next time. fortunately i have a nice backup... a lovely workshop which will do my nasty design into reality

and here is the servo i got from the market (in the attachment also), with D shaped shaft, r: 4mm.
those servo are 100kgfcm (10Nm) and 180kgfcm (18NM), daisy chain, digital, the seller claimed that the servo can give the feedback about bla bla bla which i just need the current angel position. but the servo don't come with a specific built controller instead of normal H bridge or kind of servo shield for arduino uno or mega. not sure.
*in short: i don't know about how to choose the correct position sensor/solution
**sure... the seller is cute by saying "ouw... our servo is accurate, digital, digit, can connect to other servo like daisy chain and u can call each servo by each ID". honesty, i don't have experience on this but still it seems this kind of servo is my best bet to go for this project. what do you think? any opinion would be apreciated. i'm a beginner.

anyway, i have basic several questions.

1. arm joint construction. it seems i have problem. the servo looks wow with that torque but after i did my simple calculation, gosh... i need even bigger torque around 50NM or even more if i use longer arm. should i use pulley and belt solution with let's say 10:1 ratio? then how can i detect the recent angle of my arm as this is my main concern for robot arm. that's really painful. any idea about this? 180kgfcm using belt and pulley 10:1 so i can have kgfcm in the shoulder? perhaps there is some generous guys here want to share a bit about belt and pulley implementation as i'm absoluetely zero in this field. in my dummy head, it's only "attach the servo with horn, then screw it in one side of the arm". that works if the arm is small but just wondering how to synch the rotation between right side and left side of the arm chassis? using rod? pipe? shaft? i don't even know the name of the components to move the joints.

2. can i move the robot arm manually by hand, for teaching? so i can record the position and save it for certain movement set. i see servo.read() can only return the previous position but not the actual angle if i move the arm manually by hand, even when the power is on (power must be on coz i need to keep the arduino alive and my best bet is i move the hand and arduino record it if i press kind of save button )

that's all guys. please share your idea. i'll use arduino due at 3.3V for the sake of more computing power and not sure, i'll bet for AC servo with 1 controller for each servo. but i don't have any idea how to wire/connect all 7 controllers to 1 cute tiny arduino. i wish i can stick with my first plan and go with that.

thanks guys in advance and pls help for opinion, strategy like position sensor, or anything.

thanks markt,

thx alot for the link. i have many designs now

however, i'm still confusing about torque issue in the shoulder. it's not that easy like when building robot in small size like we can see in many 6 axis robot arm (check in http://www.aliexpress.com).
it's just like put servo in each joint, the start adding command like "serwo.write(90);" and go.

do u think i have to use pulley and gear so that tiny servo can move the shoulder with heavy F?
what is the formula? 60 step pulley on servo connect to 600 steps gear and i can have 10x bigger torque to move the shoulder? or simply searching for bigger servo and connect it just in one side of the arm (either left side or righ side and expecting the whole shoulder is moving as long as the T is big)?

please kindly advise how to move a component.

many thx

do u mean this?
this is still a servo from RC & toy category although it has 180 kgfcm torque and come with cheap, around $34-38. u can bargain in aliexpress from $62 and i got $34 just because i keep typing using google translate from english to chinese
this is the picture of wiring coz i got banned or rejected when uploading this file
wiring from controller to arduino

that's how to wire to arduino board.

sure if this servo is used on that robot arm toy, even just one servo, put it on the left or right side of the shoulder, it won't be a problem at all.
but i'm sure there is a better way to connect one servo to move the whole arm/shoulder part,and i don't have any idea about that, probably:

• about belt and pulley to make the torque bigger and strong enough to lift the shoulder
• big gear put it on the left side where servo is attached and connect it using kind of rod or anything to the right side chassis of the shoulder

not sure... but yes...i have problem for that big required torque on shoulder.

ah... about moving manually using hand, perhaps this short video will give u better understanding about what i mean, teaching the arm to move in certain motion using hand and just save it the position/angle of all servo in every stop.

this is the expensive example
teaching arm using hand in tokyo

and this is the cheap example which fit into my pocket and wallet
teaching robot arm in cheap and affordable cost

simbawave:
do u think i have to use pulley and gear so that tiny servo can move the shoulder with heavy F?
what is the formula? 60 step pulley on servo connect to 600 steps gear and i can have 10x bigger torque to move the shoulder? or simply searching for bigger servo and connect it just in one side of the arm (either left side or righ side and expecting the whole shoulder is moving as long as the T is big)?

please kindly advise how to move a component.

many thx

Yes, reduction gearing increases torque at the expense of speed. You also increase losses due to
friction so that needs to be factored in as it reduces effective torque somewhat.

Most servo units have a fixed speed of about 50rpm, so are seldom very high in torque. For moving
a large arm you'd want at most 10 rpm (otherwise its dangerous), and that factor of five in torque
would be useful.

As I said the more you can reduce the max torque requirement by clever design, the easier the mechanical
drive side of things will be.

Want more information on Strain Wave Gear? Feel free to contact us.