Splines are necessary for many reasons. In addition to transmission of torque, spline profiles are used to find and centralize the axis between a splined shaft and a splined hub located in the mating part. While it is possible to achieve this result with major diameter/minor diameter fits of the mating splines, the true centering effect can only be achieved by using involute splines. This involute, or “side fit” profile is the most common used spline in production today. While the geometric form is complicated, modern hobbing, grinding, rolling, and other forms of profile generation make this type of spline easily obtainable, even in a high volume production environment. Also, the ever changing spline specifications defined in ANSI, DIN, and ISO make the inspection of the spline a rather unique challenge as well. The most important task in creating and inspecting these complicated involute forms is “knowledge” about the production and quality control of such splines. Following is a small snippet of information which is taught in the Frenco Spline Technology class. With more than 25 years committed to the understanding of the spline, Frenco teaches the world’s only technology class exclusively devoted to spline understanding.

Side fit profiles obtain both the centering and the torque transmission with the tooth flank contacts. Under load, the centering effect is independent of the torsional clearance of the internal spline to the external spline. However, in a no-load condition, clearance between the internal and external profiles occurs and the resultant centering effect is lost with direct relation to the amount of clearance.

For this above reason, it is desirable to have as little clearance as possible, creating a close fit between tooth and space. To get this effect, close manufacturing tolerances must be maintained. In practice, however, standard production processes produce an ever-increasing fit clearance over time. In special cases, a negative fit clearance in the form of an interference fit is required. In production, the amount of interference is very difficult to control and will be subject to the same fluctuations as a clearance fit (see Figure 1).

Figure 1

Contact Area

Of all form fitting connections, splines are among the most difficult to calculate and predict. For example, a standard 1.00″ spline with 24 teeth has 48 individual lines of contact. When an internal and external spline each having 24 teeth are inserted together, the design theory is to have any equal symmetrical fit clearance at all 24 teeth. However, inspection of the mating profile systems shows some space to be slightly smaller or slightly larger than others. The smallest widths of the internal spline are entirely responsible for the efficiency of the entire spline system. The equal distribution of size and form fluctuations within both profiles directly influences the number of contacting tooth flanks under load. For clearance fit designs, this number is not important. However, on in reference (force) fits, the line of contact at the tooth flanks has an enormous effect on the necessary force required during assembly. A poor line of contact influences performance of the spline as well as increases fatigue of material. As a rule it is desirable to have a good line of contact, and this can only be obtained by designing and manufacturing splines with little size and form deviations within the profile.

Effective Spline

In rare cases, when an internal spline is mated with an external spline, the quality of fit may resemble a cylindrical (non profiled) fit. In “side fit” profiles, this fit is achieved through perfect contact of the tooth flanks with the spaces. The same applies to the system basic sleeve-basic shaft where an absolutely round and cylindrical bore or shaft will never be possible, likewise, a spline will not be absolutely round or equally cylindrical over its entire length. Production is responsible for uneven form where irregularities will always exist.

Not only the size but also the existing form errors are important for the clearance of the fit. The amount of influence of size and form to the clearance fit is different on various contours.

With regards to cylindrical (not profiled) fits, the actual size of the components determines the fit much more than the form.

Form errors reduce the effective size of bores and increase the effective size of shafts. Cylindrical fits always have form deviations, however they are not as big as for splines. If accurate round fits are requested, they will be ground after heat treatment. The grinding of a round geometry is an acceptable and economic solution.

The cost of grinding splines is prohibitively high and is a process that is usually avoided. Even with the need for hardened structural parts, a rework usually will not follow heat treatment. At the time of production of soft (green) splined parts, big form deviations arise. Additionally, heat treatment makes the deviations of contour worse. The effective tooth thickness and space width are greatly influenced by these factors. The effective tooth thickness and space width are termed the “effective spline.” (See Figures 2 and 3)

Figure 2
Figure 3

Fit System Actual-Effective

The actual (real) measurable size of tooth thickness and space width at the pitch circle diameter (PCD) is called “actual.”

The max. material tooth thickness and space width measured by the metal to metal fitting mating part without any form variations is called “effective.”

The interference effect of all form variations cause a change of the tooth thickness and space width regarding the fit clearance. It increases the tooth thickness of external splines and decreases the space width of a internal spline in regard to the fit clearance. The most important form variations are: Profile variation, Index variation and Lead variation. In addition to these primary variations, following variations will exist as well:

  • Concentricity error
  • Torsion (twist, distortion)
  • Damage
  • Eccentricity
  • Dirt contamination
  • Surface finish deviationThe summation of all of the single form variations can only be determined by metal to metal fitting of an “ideal” mating part (go gage).Unlike cylindrical fits, the manufacturing tolerance and the form tolerance are distinguished separately on splines. The manufacturing tolerance is the tolerance of the space width and tooth thickness at the circular pitch diameter. This is a required measurement for the adjustment and wear of tooling. The common designation for this specification is “actual” tolerance and from this the size “max. actual” and “min. actual” are derived.Internal Spline: The maximum actual tolerance limit is converted to the measurable feature “dimension between balls or pins.” The maximum actual auxiliary mark is necessary for manufacture control. The minimum effective tolerance limit controls the smallest fit clearance and is checked as an attribute with a composite go gage plug.

    External Spline: The maximum effective tolerance limit controls the smallest fit clearance and is checked as an attribute with a composite go ring gage. The maximum actual auxiliary mark is necessary for manufacture control. The minimum actual tolerance limit is converted to the measurable feature “dimension over balls or pins.” (See Figure 4)

    Figure 4

    Spline standards allow the use of sector not go gages in place of measurement between/over balls or pins. The method of gaging the size, however, is not accurate by 100 percent. Size measurement must see as little form variations as possible. Not go sector gages will check profile errors as well. The dimension over/ between balls has the highest priority for actual inspection methods.

    To show the tolerance zones, the tolerance chart seems to be most suitable. The tolerance limits are shown in Table 1.

    Table 1

    The true understanding of a spline tolerance system is graphically displayed in a bar graph. In this type of graph, it is possible to quickly view the spline tolerance limits of the “effective” (form) and “actual” (size) sizes, as well as the manufacturing tolerances used in setting up the manufacturing machinery. This knowledge also aids in the selection of the proper tooling and inspection equipment. Knowledge of spline terminology also makes communication between vendor and customer a much more comfortable procedure. It is human nature to shy away from conversations in a subject you are not comfortable with. By learning the basics of terminology, toleranceing limits, and geometric spline configurations, it is possible to be very comfortable in discussions about spline. There truly is no “black magic” when it comes to producing and inspecting splines if you simply commit to arming yourself with a little basic knowledge.