Bal-tec™ Home Kinematic Hinge
The Kinematic hinge is an important page in the catalog of designs. The first step in picking a hinge design, with the best features at the lowest cost is to outline the required functions. What loads must be accommodated, what accuracy must be met and how many degrees of freedom must be constrained?
If freedom in all three rotary motions is desired, or acceptable, a sphere and a cone or one of its derivations would be the choice. Which one of the three constraint devices chosen will be determined by the required accuracy, the load carrying requirement and the financial budget? (See the discussions of the properties of the different couples in another area of this paper)
In situations where roll is the desired motion and one additional linear degree of freedom is acceptable, two tandem, two degree of freedom constraints will be the design to choose. Two, two ball Kinematic couples with a common cylindrical element will form an outstanding hinge. The two vees formed by inclined cylinders are a higher load- carrying alternative. Two mechanically connected spheres cradled in two vee blocks or one of its derivations will give the same results.
When only one rotary degree of freedom is tolerable, we will need a combination of the last two devices. By mechanically connecting two rotating spherical elements together and matching them with a conical cup and a vee block, a device with a single degree of rotary freedom is created.
The choice of materials for a hinged Kinematic coupling can be much more critical than for a standard static coupling. If there is frequent exercise of the hinge function, the high unit loads placed on the surfaces of the couple as a condition of Kinematics, will result in accelerated wear and a serious problem with fretting. As already discussed, the practical choices come down to ceramics and cemented metallic carbides.
Due primarily to cost, availability and machineablity the material of choice is cemented Tungsten Carbide. In situations of very limited exercise of the hinge, the standard high chrome, high carbon, hardened stainless steel will suffice. With a mastery the Kinematic hinge comes some really unique design opportunities.
Building very stable, repeatable, vertical Kinematic couplings that do not require heavy preloading is challenging. By placing a single degree of freedom hinge consisting of vee a cone and two spheres in a horizontal position at the top of the platform and a sphere against a flat, as far down at the bottom as possible, outstanding performance can be achieved.
This isn't much of a dissertation on a subject that has received wide discussion in the literature; but this one golden design with variations in the hardware and the materials used is just about all that is needed. In order to accommodate heavy loads, components with large radii will be required. In this regard compact components, such as the quarter round and the rose bud (both discussed in the Kinematic Encyclopedia paper on this web site) will be the ideal choices.
Brute force vertical Kinematic couplings have their place in the scheme of things. When the payload is light, any of the conventional Kinematic couplings can be used vertically. What is required is an adequate preload in the form of mechanical or magnetic force to hold the two platforms into intimate contact with each other while they are in the vertical position. Both of these forms of preload are discussed in other areas of this web site.