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Boat Hardware Installation
Hardware installation methods, problems and principles of Bonding
Boat Articles, Guides, Commentary and archival articles & helpful information
Need a professional advice - methods, principles and tips to hardware items installation on wood surface
Hardware installation on wooden boats presents many problems. Even the smallest boats are usually equipped With hardware that must be screwed or bolted to the wood surface. Unfortunately, hardware attached by normal methods has created a multitude of problems which reveal some of the weaknesses of wood. The low density of wood makes it an ideal hull construction material, but, unfortunately, this low density is a weakness where hardware is concerned. The extreme high point loadings that hardware can generate can easily crush the wood fiber.
Typically, high load-hearing hardware is attached to a deck surface by through bolting with nuts and washers used on the underside of the deck to clamp the hardware item firmly to the wood surface. Depending upon the size of the washers used, you can apply only a minimal clamping load on the nuts without quickly reaching the crush strength of the wood fiber. If the predrilled holes for the through bolts are the least bit sloppy, it is likely that the hardware item on the deck will move slightly, depending on the amount and the direction of the loads applied. To prevent the entry of moisture underneath the hardware and into the bolt holes, various forms of bedding compound are used as sealants. Over a period of time, however, the continued movement of the hardware item tends to overcome the elastic resistance of the sealant, which eventually allows moisture to infiltrate the wood im- mediately surrounding the hardware installation. This invading moisLure contributes to a general weakening of the wood fiber and sets the stage for dry rot spore attack. Unless you detect dry rot damage early, and remove and rebed the hardware item, serious structural damage can take place, possibly resulting in total failure of the hardware item.
To overcome these problems, we have developed a different approach to hardware attachments. We refer to it as hardware bonding. As the name implies, hardware items are bonded to wood fiber with the goal of distributing the hardware load over as large an area of wood fiber as is possible.
There are two distinct ways to achieve this. The first is to bond all fasteners (screws, bolts, or threaded rod) involved directly to the sur rounding wood fiber. The second is to bond the hardware item itself to the immediate wood fiber on which it rests. Using proper hardware bonding installation techniques, as explained in detail in this chapter, you can improve hardware load capacity dramatically over that possible with standard hardware installation methods.
WEST SYSTEM resin is an excellent bonding agent with wood. In addition, you can also use west system resin for metal bonding, either metal-to-metal or metal-to-wood. It is this capacity to bond dissimilar materials that makes hardware bonding with WEST SYSTEM resin practical.
Principles of Fastener Bonding.
You can accomplish the fastener-to-wood bond in a number of ways. The easiest and most common method is simply to wet out a standard size pilot hole for a given fastener. This forms a wood-resin matrix immediately surrounding the fastener that is much stronger than the wood by itself and distributes the fastener load over more wood fiber area. A fastener that is bonded in this wood-resin matrix has a much greater load capacity than the same fastener inserted in the same size pilot hole, but without resin saturation. (See Figure 2.) To make such a fastener-to-wood bond, use a pipe cleaner to work the standard resin mixture well into the pred rilled pilot hole, wetting out all of the wood fiber immediately surrounding the hole. Then insert the fastener in the hole and allow the resin to cure.
You can further improve the metal-to wood load transfer with fasteners by increasing the amount of resin that surrounds the fastener. Here you are taking advantage of the fact that West system resin has much higher density and strength than wood fiber itself. To increase the amount of resin that surrounds the fastener, drill an oversize hole. This may be much larger than the fastener twice fastener diameter, for example. Drill the wood to the proper depth and then fill the hole with standard resin and set in the fastener. After the resin has reached a cure, the resulting matrix of fastener-resin-wood provides an increase in surface area of wood fiber available for load distribution. As a result, the fastener is capable of withstanding more load before complete withdrawal failure. In almost all situations with this composite, failure occurs either within the fastener itself, or between the resin-wood bond. The bond between the fastener and resin is rarely a failure point because of the excellent keying action of the fastener threads into the resin matrix. Generally, this keying action is considered sufficient so that you need little preparation of the surface area of the fastener itself to promote a better fastener-resin bond.
Figure 1 - Tension withdrawal
Figure 1 illustrates the effect of surface area on small fasteners installed by the resin bonding technique. It shows the results of a series of tests we conducted. We used 1 1/2 inch long wood screws, ranging in size from No. 8 through No. 14, and drilled oversize holes from 3/16 inch to 3/4 inch in diameter for all of the screw sizes in the samples. The samples were Sitka spruce with 1/4 inch plywood bonded on to represent the deck and blocking. The first column gives the load values that one could expect from a standard wood screw inserted into its proper size pilot hole with no resin added. The succeeding columns give the values for the same screw inserted in increasingly large oversize holes.
The No. 12 wood screw, when inserted in a standard size dry pilot hole, required 901 pounds to withdraw. The same screw bonded into a 14 inch diameter oversize hole required 1,332 pounds to withdraw. The same fastener bonded into a 3/8 inch diameter hole required 1,697 pounds to withdraw.
These results reflect the increase in surface area between the two hole sizes. It is significant that the same fastener, when bonded in a 7/16 inch diameter hole, failed in tension at 1,742 pounds. Thus, for tension withdrawal loading with No. 12, 1 1/2 inch long wood screws, there is no reason to use an oversize hole larger than 7/16 inch diameter. At this point, enough surface area is available for load dissipation to overcome the breaking strength of the screw itself.
The No. 14 wood screw, with its larger shank size, required a 3/4 inch diameter oversize hole to reach its breaking strength of 2,746 pounds. However, the same screw in a 3/8 inch diameter oversize hole was only capable of a load of 1,606 pounds, which was slightly less than the No. 12 size screw was capable of in the same size hole.
Of major significance is the fact that all the screw sizes (No. 8 through No. 14) were capable, of about the same load capacity when bonded into the 1/4 inch diameter hole (see third column). In other words, screw size is only important when it comes to ultimate tension loads, where the screw strength itself is at issue. Otherwise, the diameter of the predrilled hole in which the screw is bonded is the controlling factor for load capacity.
Resin Saturating Pilot Holes for Small Fasteners
As we have discussed, the resin saturation of the standard size pilot holes can contribute greatly to the load capacity of a fastener. Figure 2 illustrates the results that can be expected with both wood screws and self-tapping screws of different lengths, in either dry piedrilled pilot holes or resin saturated pilot holes. (Sec Figure 3 for clarification of different screw types.)
The simple addition of WEST SYSTEM resin to the fastener wood matrix can increase the tension load capacity more than 70% over that of a fastener in an unsaturated pilot hole. We gain this phenomenal increase in capacity at very little expense or effort, and use it in a majority of moderate hardware applications.
Figure 2 - Withdrawal resistance for resin coated pilot holes vs. dry pilot holes
Figure 2 also shows the superiority of tapping screws over wood screws in both dry and resin saturated pilot holes. The tapping screw has a fully threaded shank that provides more surface area and more mechanical keying into the resin-wood matrix than the wood screw. Unless the smooth metal shank portion of the wood screw is carefully treated for bonding, the wood screw is inferior to the tapping type in tension withdrawal.
Figure 1 - Wood screw types
Large Diameter Fasteners
As fasteners yet larger in diameter (bolts, machine screws or threaded rod), the job of determining load capacity becomes more predictable. Fastener failure in tension becomes less of a problem because as fastener diameter increases, fastener strength grows at a far greater rate of increase than does the wood surface area over which the load is dissipated. The use of oversize holes and a resin interface between metal fastener and wood fiber become increasingly important with larger fasteners, especially when trying to maximize the fastener potential. The following ultimate withdrawal resistance values for resin bonded bolts from three species of wood were determined through tests run by Kurt Keidel at Ohio State University in June 1977. These values are the pounds per square inch load for epoxy bonded bolts in oversize lound holes.
Ultimate withdrawal resistance
Honduras mahogany-1,884 psi White ash-1,618 psi Sitka spruce-1,360 psi
Knowing the resistance load values of the different species of woods, we determine the load capacity for a given large diameter fastener in pounds per square inch Available square inches, of course, are figured on the diameter of the drilled oversize hole, and not the diameter of the fastener itself. Assuming that we had a 3/8 inch diameter bolt bonded into 3 inches of wood fiber in a 1/2 inch diameter hole, we could assume 4.5 square inches of surface area.
The only other thing we need to know to find the load capacity of this bolt application is the pounds per square inch (psi) that the wood being used is capab e of withstanding. Sitka spruce, for example, is capable of withstanding an ultimate withdrawal load of 1,360 psi. With a 4.5 square inch surface area, you would arrive at an ultimate withdrawal resistance (for the bolt application) of 6,120 pounds.
If white ash were used instead of Sitka spruce, the ultimate withdrawal load would increase to 1,610 psi. If Honduras mahogany were used, the ultimate load would be 1,884 psi. It should be noted that the safe allowable design loads should not be over one half the ultimate withdrawal resistance loads given above.
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