by:
James D. Logan, P.E.
President, Metriguard Inc.

The Metriguard Model 7200 High Capacity Lumber Tester (HCLT) and the CLT Continuous Lumber Tester produce MSR, MEL and E-rated lumber grades which are more thoroughly tested than material produced by a machine which does not make a direct physical measurement of the mechanical properties by application of a testing load. Pieces whose bending strength is less than the stress applied are broken in the process and removed from the production flow.

By removing a few pieces per shift in this way a significant improvement in exposure is made because the very weak pieces are the ones which cause a majority of problems in lumber applications. The manufacturer should regard every piece broken in the testing machine as a potentially expensive problem that was prevented from reaching a customer.

An extra level of quality assurance is available in MSR, MEL or E-rated lumber grades which have been produced by a Metriguard Model 7200 HCLT or a CLT Continuous Lumber Tester. This benefit is lost when non-contact methods are used to grade the lumber.

Opinion
You are dealing with a product which everyone understands will be used in a structural application - to bear a load for a long period of time. In my view, to avoid putting a load on it at the manufacturing plant for fear it might break is not logical.

MSR has been very successful in North America for the past 30 years. The product and the grading rules have evolved around the use of the CLT. Before we accept another method that leaves out some of the things the CLT does in the grading process we need to carefully examine everything the testing process is doing, whether or not it is recognized in the regulations.

Introduction
The CLT and HCLT lumber stress grading machines measure bending E by means of a rolling stiffness test in which the material is deflected downward, then upward to a prescribed deflection while the force at the center of the bending span is monitored. As a direct result of this induced deflection, a bending stress is applied in the flatwise direction. While the E-measurement zone extends along the length to within approximately 27.5 inches from the ends of the specimen, the induced bending stress extends to the very ends of the piece as shown in Figure 1, in an envelope which is defined by the E-profile and the length of the bending test span. The result is a full-reversing 100% flow, bending proof stress screening. In the attached Figure, we see an illustration of these stresses. For 1650F 1.5E, the stress is approximately 915 psi, or 55% of the design fiber stress in bending (Fb). The stress is proportionally higher for higher grades. Because the machine is used for grading lumber in thicknesses of 1.5", 35 mm, and 45 mm, and the stress is proportional to thickness, a deflection of 5/16" is chosen as a good compromise for all these thicknesses.

Beneficial effects of the stress
While the applied stress is less than that required to assure the bending strength for typical MSR grades, it does cause an occasional failure while the pieces go through the machine. This testing process effectively removes the lower tail of the bending strength distribution for all the material that flows through the machine, improving both the MSR grades and the visual grades which are produced in the same run. While MSR grading is based upon an assumed relationship between ultimate bending stress and E, this is an approximate relationship, and lumber can have higher or lower ultimate bending stress than is assumed. There are occasional "bad actors" which have a bending strength much less than the assumed value. Subjecting these pieces to an actual bending load can cause them to fail. This removes them from the process and prevents them from getting to the customer. The handling problems associated with this broken material have been virtually eliminated in the Model 7200 HCLT. Actual field experience has shown at least a factor of 10 improvement over earlier bending type equipment in clearing broken pieces without stoppage.

These "bad actors" may contain excessive localized slope of grain, timber breaks or some other cause that makes them considerably weaker than expected for a given E value. Some of them don't wind up in the MSR grades, but these pieces can produce just as devastating and expensive consequences by failing in service as they could if they were in the MSR grades.

Effects arising from changes in E
The instantaneous maximum stress (smax) applied is a function of the E, the thickness of the material, the span length and the bending deflection. For a typical situation with 1.5", the span is 48" and the deflection set to 5/16", the stress is equal to 6.1035x10-4 *E, where E is the modulus of elasticity in PSI. The average E for 1650F 1.5E lumber is at least 1,500,000 psi, so on the average, a stress of at least 916 psi is applied. If we use a Low-Point E threshold setting of 1.2 million psi, the minimum (average of upward and downward) stress applied to that particular part of the piece will be (1.2/1.5)(915.5)=732 psi, keeping in mind that the Low-Point E setting for that grade may vary from mill to mill for a given grade. The applied minimum stress may be higher than this number, however, because as illustrated in the drawing, the stress applied at a sharp local minimum in the E profile can be higher because of the higher E material on either side of the minimum.

Warp Effect
The amount of stress applied is influenced by the straightness of the piece. If the piece is bowed upward, then the downward-applied stress will be increased and the upward applied stress will be decreased by equal amounts. An estimate of this influence may be made by examining detailed plots of the bending forces as material is fed through the machine. We have found that the warp effects can on occasion amount to 50% increase & decrease in applied stress, but are more typically confined to a range of 0-20%.

Options
If an increased stress level is desired, the Metriguard 7200 HCLT machine may be set up with an increased deflection, thereby increasing the stress level. This involves changing the angle of the clamp roller assemblies and vertical position of the load rollers. Making similar adjustments to a CLT could also be done but with considerable difficulty.

Costs
Costs arise from having to trim the broken pieces resulting in some trim losses and selling a small portion of the material as a lower value, shorter visual grade. Down time can result if a broken pieces jams in the machine. Down-time cost is greatly reduced in the new Metriguard Model 7200 HCLT.

For Further Information Contact:
Metriguard Inc
PO Box 399
Pullman WA 99163
Phone 509-332-7526
FAX 509-332-0485
e-mail sales@metriguard.com

  Go to - [home page] - [contacts] - [products] - [literature]