Coupler Size: Prototype and Model, by Bruce A. Metcalf

Model Couplers

The NMRA has established Standards for coupler (centerline) height for sixteen of the established scales in NMRA Standard S-1). They have also established Recommended Practices for (dummy) coupler body size for seven scales in NMRA Recommended Practice RP-21.2. Using the drawing in RP-21.2 to judge the ratio of body size to knuckle height, estimates of approximate knuckle heights were made (Figure 8). All of these dimensions are shown in Table 1.

Figure 8: NMRA RP-21.2 Solid Coupler

While commercially available couplers are manufacturered for most modeling scales, many are not accurate scale reductions of even the largest prototype design. Fortunately, the wide variety of couplers and mounting arrangements available today make it possible for the prototype modeler to make a more accurate selection that will enhance the appearance of their models.

To select the right size coupler, the careful modeler thus needs to know not only the scale, but the date and gauge of the prototype, and often something of the history of the specific line modeled. Sorry, some simple questions just have complex answers.

Kadee Quality Products long ago established the de facto standard coupler in most of the modeling scales for which they produce products (and several in which they do not). In 1990, the company split into two indepenent parts, with the Kadee name retained by the part that produces parts for HO and larger scales (including "G" and HOn3), and a new company called Micro-Trains Line taking on all products in scales smaller than HO (including N and Z).

The recent expiration of critical Kadee patents has opened the market to a number of new manufacturers, especially in HO scale. Accurail's contribution, sold as Accumate™, is the first of these to provide me with sufficient data to be included. I have also obtained data on one style of McHenry couplers. Other manufacturers of compatible couplers (and interested modelers) are invited to send along data on knuckle height, and it will be included as quickly as time permits. Because of their interoperability, I have chosen to discuss all brands of couplers together.

Currently available couplers for the various scales, including the scale knuckle heights of each model (in the scales for which they are labeled) are shown in Table 2.

To select an accurately scaled coupler for any given model, first research your prototype and decide what size coupler you are going to model. There are basically four choices:

• Type E, 11" knuckle, for equipment built after 1932 (Types F and H are not considered here);
• Type D, 9" knuckle, for standard gauge equipment built before 1932 and heavy-duty narrow gauge service;
• ¾MCB, 6¾" knuckle, for most 3-foot gauge and heavy industrial service; and
• ½MCB, 4½" knuckle, for 2-foot gauge and light indistrial service.

Table 3 shows the size of an accurately scaled knuckle for each scale. Use this table to select dummy couplers and to compare other brands of couplers.

Table 4 shows the model couplers that are the best knuckle-size match for each scale and prototype knuckle size. Where multiple choices are shown, all couplers are within 10% of the correct scale knuckle size, listed largest to smallest.

The gray (or blank) areas in the table above reflect scale/kuckle combinations where no model coupler is within 50% of scale size.

Note too that a listing on this chart does not guarantee that these couplers will inter-operate with those labeled for the scale or gauge. I have personally found that the Kadee HO and ¾HO couplers work very well together, and others have reported that the Kadee HO and S couplers work well together.

The many different "scales" of Kadee and Micro-Trains couplers do interoperate with one another, with some challenges in magnetic uncoupling in particular. As other brands were designed to work with the Kadee design, I would expect them all to work with the same scale Kadees, but not necessarily with each other.

If it seems that very few of these products are scale reductions of prototype size, please remember that this product line goes back to the early 1950s when scale accuracy was less urgent than the ability to operate. Modern desires for more accurate sizes are restricted by the need for backwards compatibility, and shouldn't be counted against the good intentions of the manufactuers.

You will have to modify the trip pin length whenever you use a coupler designed for a different mounting height, especially if you want to use magnetic uncoupling. Remember, coupler height is an NMRA Standard for each scale and gauge, and does not change when you change couplers.

When using couplers in a larger scale than that for which they were designed, you should also be aware that the gathering range of the couplers may be limited, and will certainly be less than the larger couplers designed for the scale. This makes it even more important than usual that the centering springs are correctly installed and working properly.

You may also find that delayed uncoupling is somewhat less reliable, especially where substantial changes in trip pin length have to be made. (Of course, this isn't a prototypical activity anyway … on most lines.) Remember that even a perfectly accurate model will be unsatisfactory if the couplers don't work!

Notes:

1. This scale is not presently a recognized NMRA Standard.
2. NMRA Standards have not yet been established for this dimension.
3. NMRA Recommended Practices have not yet been established for this dimension.

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Copyright © 1994-2004, by Bruce A. Metcalf, who would very much appreciate your comments. Updated 6 November 2005