It's a fact of life, well at least in respect to RC helicopter design and operation that the almost universal use of the ball link in most all model helicopter control systems is taken as 'granted'! Manufacturers of RC model helicopters make their own ball links to similar specifications, generally with a manufacturer name, a logo/symbol or other identifiers on an appropriate surface on the link. Whilst the various links can differ, the links still have an overarching similar design and features that require an acquired respect and understanding in order to establish a long successful operational life of your RC helicopter.
The longevity of your RC helicopter depends, amongst other factors, on the ball links(and push/pull rods). Properly fitted and sized links enhance greatly the success in a better flying RC helicopter.
Now, there's some tools which will necessary to accomplish efficient practical manufacturing,assembling/disassembling and sizing of ball links and associated push/pull rods.
Ball Link Pliers, HobbyKing variety shown below:
Clevis installation/removal tool, HobbyKing variety shown below:
Generic variety shown below: A push rod / pull rod (control rod) setup tool for accurate rod length
Or failing the opportunity of having a ready made specific tool shown above, vernier (digital) calipers, shown below, can be used*:
(remember to subtract the diameter of one ball link to establish push/pull rod centre length!).
Ball link sizing tools, the items shown below are 4mm and 4.7mm, HobbyKing variety shown below:
(Sizing the link can be established by carefully rotating the ball link sizing tool in the link. Do not be too heavy handed or damage can result with it becoming unusable. Further details are noted below)
Some examples of ball link types shown below:
In general, the lettering, symbols or markings on the ball link surface face away from the ball. The back side hole can sometimes be slightly bigger than the front side hole. Check the OEM instructions to be aware of any specific assembly.
Captive links shown below:
Corresponding male component varieties of a ball link shown below:
Righto, down to the nitty gritty.
The Ball Link:
Sometimes simply called a link or 'rod end'. The external 'female' component is often made of plastic or composite material although high end larger ball links can be manufactured from aluminum or similar. The ball link is designed to screw onto a metal rod at one end and connect to the control ball on the other. The threads can vary between different manufacturers. Two ball links, attached to each end of a metal rod and sometimes carbon solid rod or tube, make up a push/pull rod.
Connecting the ball link attached to the servo arm of a particular servo on the helicopter to the ball link on a flight control mechanism be it to the main rotor head or anti torque rotor(tail rotor), push/pull rods transfer both the motion and torque of the servos to the flight control system. Some helicopters use a single push/pull rod for control, while others use two push/pull rods connected to opposite sides of the servo arm/disc to obtain both push and pull control mechanism as the servo arm or disc rotates. The benefit of a push-pull control system does balance/distribute the loads to each side of the servo output shaft, as well as creating an advantage within this design adding control redundancy if one push/pull rod fails.(You can never have enough redundancy!).
Push/pull rods are designed to cope with the dynamic loads placed on a specific control system function. Small, lightweight push/pull rods are generally used on the smaller electric helicopters, while larger, heavy-duty(piano wire or carbon) pushrods are used on large IC, electric, and turbine helicopters. Regardless of duty cycle, material or what the size, the push/pull rods all contain a similar appearance and have two ball links(generally) connected by a metal rod. The metal rods and ball link ends all use common standard threads to connect, eg: 2-56, 2mm, 4-40, 3mm etc. An OEM helicopter kit , be it ARF or otherwise will have a variety of metal rods(varying lengths), with sufficient ball links to make the needed push/pull rods.
Constructing pushrods should not be seen as difficult, however, as with many facets of an RC helicopter build, there will be skills, experience and technique that will result with the job being easier and result in a safe well constructed connection.
Ball links, whether being the female or male component, can initially present a tight fit on the push/pull rod and or servo arm, and be difficult to initiate the tread starting. However, you can provide a 'tapered' start to the end of the ball link by carefully using the point of modeling knife to remove a slight portion of the hole edge. Be very careful to remove only the smallest amount needed to start the thread. Also examine the thread start on the rod for damage or burr. Correct if necessary.
A clevis installation/removal tool(shown previously) makes light work of(and the threat of damage) threading the ball link onto the rod. The tool slips over the end of the link allowing a greater 'purchase' of the link so it can be turned so much easier as most OEM ball links are manufactured to provide a firmer connection with the rod. Each ball link needs to be started on each end of the control rod, with the rod then held with a stout pair of pliers or in a vice and then screw the second ball link on with the installation tool.
The OEM of the helicopter will provide the required lengths of the push/pull rod, generally measured from the ball centres.(setup tool methods shown previously) Whatever method used, make sure that each ball link is screwed on the rod an equal distance. This will be evident by the amount of remaining thread shown on the rod. As a habit, try to take note on how much engagement of the thread is within the ball ink so to not end up with enough thread engagement which can result in ball link disengagement and unsafe operation and consequences.
Generally, installing a pushrod may seem to be straightforward, however acquired skills, experience and technique will assist in producing a safe competent connection and avoid possible unsafe consequences.
As noted previously ball links come in varying shapes and sizes, most all are designed to snap on the ball in one direction. The OEM assembly directions should indicate how to determine what side of the link faces the ball. However, if it is found that no information is available regarding assembly e.g. purchased a second hand helicopter, generally any type of lettering, symbol or marking should face the outside of the ball. What's that you say , no markings, well then careful examination, under magnification is helpful, to ascertain any differences between either hole side and careful test fitting of the ball link may provide for the proper orientation of the link. Failing that, or any doubt, then maybe replacement to other OEM parts(with noted orientation) need be considered. Remember, the thought of the links coming apart in flight is enough to remove the doubt and act accordingly .Remember: Safety is no accident.
Tools - Whilst 'snapping' the links on/off the ball by hand is obviously possible, there are many ball link tools on the market (shown previously) to make this job easier. Ball Link Pliers, are designed to both install and remove the link from the ball. These tools have a lower long, slotted curved jaw that easily reaches under the ball with the associated upper jaw comprising an appropriate sized anvil.
Assemble each link onto its respective ball, move the link and check for excessive friction between the link and the ball. The push/pull rod linkage should move freely under its own weight with no outside assistance. Any friction could be caused simply by tight manufacturing tolerances or a slight deformation from the manufacturing process. If apparent, the friction will present a power drain from the servo and can also produce imprecise control of the helicopter as the link moves with varying friction to a different position on the ball. Here are some techniques to reduce/eliminate ball link friction:
The simplest, easiest technique is to lightly/carefully squeeze the sides of the ball link with pliers while it’s on the ball. This slightly deforms the ball link and reduces any internal stress within the material, providing a smoother fit to the ball.
Use a ball-sizing tool(shown previously) to remove some of the material inside the ball link hole. The tool is adjustable, so can remove a slight or larger amount of the ball link before refitting to the ball. It’s always better to adopt a "remove a small amount" rather than remove too much in one instance ending up with a unusable ball link. Remember: Remove little, check a lot".
Another technique is to apply a lubricant(non damaging grease/petroleum jelly or similar) on the ball and link. The lubricant, after several flights will wear the ball link changing the fit of the connection. However, do be aware of contamination with dust and other particles , especially during take off, hover and landing and check the connections frequently. There is a point where the lubrication should be removed to prevent undue wear of the connection.
Generally, there is no need for thread lock to be used on ball links although the ball may be required to be 'locked', especially when situated in rotating circumstances(rotor head and similar). Servo arm installations may require additional use of thread lock associated with lock nuts. Whatever the case ALWAYS refer to the OEM assembly directions.
Maintenance, repair and further unit life consideration
Ball links will wear out, the components have a finite unit life, and must be replaced during regular periodic helicopter maintenance. If there is any doubt regarding a particular flight control mechanism then always examine thoroughly and replace ALL suspect items. Ball links are used in very high-stress areas, and can be easily damaged during flight operations including transporting to and from the flying field (hangar rash). Always establish a habit utilising good visual inspection, also carefully push/pull on the ball link checking the integrity of the connection. As noted previously, replace the ball and link if there is any doubt. Competent, safety aligned maintenance habits(including post flight checks) will ensure a long operational life of the helicopter for many years.