Patent Grant
8499665
The present invention relates to manually operable impact tools and, more particularly, to provisions controlling the transmission of torque from an impact head to a user-engageable portion of the impact tool.
Many tool handles, such as hammer handles, are constructed of a metal, a synthetic or a composite material. Steel and fiberglass, for example, are often used for tool handle construction. These materials offer reduced materials cost, uniformity of structure and the ability to securely and permanently affix the hammer head or other tool head to the handle. Metal, synthetic and composite handles are relatively durable as compared to wooden handles. Metal, synthetic and composite handles have some disadvantages, however. These handles tend to transfer torque (twisting about the longitudinal axis of the handle) and kinetic energy to a user's hand when a workpiece is impacted. Many hammers with metal or synthetic handles are provided with rubber or rubber-like sleeves at the free end opposite the hammer head to provide a degree of impact protection for the hand of the user. Most of these sleeves are constructed of a relatively hard, non-cushioned single material, however, and provide little or no damping. In addition, such sleeves are not engineered to address torque or torsional force applied to the user's hand that may result when the hammer head “offstrikes,” for example, when the head face misses the intended target, and the side of the head hits a structure such that the impact tends to twist the hammer about a longitudinal axis of the hammer handle. U.S. Pat. No. 6,370,986 (of same Assignee as the present invention), hereby incorporated by reference in its entirety, discloses a hammer with a cushioning grip. It has been found, however, that the teachings of this patent do not sufficiently address torsional or twisting forces imparted to the hammer during impact. A need exists for an impact tool grip that can be used on metal, composite and synthetic handles that provides a high degree of torque absorption and cushioning to reduce the kinetic energy transferred to the user's hand during impact and that can be applied to these handles easily during the manufacturing process.
In accordance with an embodiment of the present invention, a manually operable impact tool is provided that comprises an elongated handle and an impact head disposed at one longitudinal end portion of the handle. The handle includes an internal core structure and a cushioning grip disposed over the core structure. The cushioning grip includes an inner layer of thermoplastic rubber having a Shore A durometer in the range of about 10 to about 40, and an outer layer of thermoplastic rubber disposed over the inner layer and having a Shore A durometer in the range of about 55 to about 90.
In accordance with a further embodiment of the present invention, a method is provided for making a manually operable impact tool. An elongated handle is provided that has an internal core structure. An impact head is disposed at a first longitudinal end of the handle and a portion of the core structure is covered with a first layer of thermoplastic rubber having a Shore A durometer in the range of about 10 to about 40. The first layer of thermoplastic rubber is then substantially covered with a second layer of thermoplastic rubber that has a Shore A durometer in the range of about 55 to 90.
In accordance with a further embodiment of the present invention, a manually operable impact tool is provided that comprises an elongated handle and has an impact head disposed at one longitudinal end portion of the handle. The handle includes an internal core structure that has a tuning fork portion. A cushioning grip is disposed over the internal core structure and includes a soft inner layer of a solid, non-foamed thermoplastic rubber and an outer layer of thermoplastic rubber disposed over the inner layer. The outer layer is harder than the inner layer.
Objects, features, and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.
The above-mentioned and other features and advantages of the present invention, and the manner of attaining them, will become more apparent and the disclosure itself will be better understood by reference to the following description taken in conjunction with the accompanying drawings, wherein:
The present invention will be described with reference to the accompanying drawings. Corresponding reference characters indicate corresponding parts throughout the several views. The description as set out herein illustrates an arrangement of the invention and is not to be construed as limiting the scope of the disclosure in any manner.
The manually operable impact tool 10 includes an impact head 12 (which is not cross sectioned in
The impact head 12 for the hammer shown is of conventional construction and is preferably made of steel or other appropriate metal, formed by forging, casting, or other known methods. The impact head 12 includes a striking surface 18 and optionally may include nail removing claw 20.
The internal core structure 14 is a rigid structural member that supports the impact head 12. In one embodiment, as shown in
The internal core structure 14 shown in
The inner layer 22 may be a TPR having a Shore A durometer in the range of about 10 to about 40. The inner layer 22 more preferably has a Shore A durometer of between about 30 to about 40. In one embodiment, the inner layer 22 has a Shore A durometer of about 35. The outer layer 24 is relatively harder in comparison with the inner layer 22 yet may still be flexible or resilient. The outer layer 24 may also be a TPR, and in one embodiment is the same type of TPR as the inner layer 22 so as to ensure a chemical and melt bond between the two layers. The outer layer 24 may alternatively be a different type of TPR than the inner layer 22. The outer layer 24 has a Shore A durometer in the range of about 55 to about 90. In a more preferred embodiment, the outer layer 24 has a Shore A durometer of between about 55 to about 65. In one embodiment, the outer layer 24 has a Shore A durometer of about 60. The higher durometer of the outer layer 24 lends to increased durability and decreased wear characteristics. By separating a higher durometer outer layer 24 from the internal core structure 14 with the lower durometer inner layer 22, improved torque control and vibration damping effects are realized.
One skilled in the art will appreciate that the exterior impact-cushioning gripping structure 16 can be formed on the internal core structure 14 using well known, conventional molding processes on a conventional two part or “two shot” molding machine, as described in U.S. Pat. No. 6,370,986, referred to above. The layers may, alternatively, be successively pressed on (inner layer, then outer layer). It is desirable to have different wall thicknesses at different parts of the gripping structure 16 because the butt end 34 of the gripping structure 16 may be subjected to repeated impacts, so in one embodiment the bottom wall 36 of the gripping structure 16 is thicker than the side walls 38. In one embodiment, the side walls 38 are relatively thin to improve the feel of the gripping structure and to provide improved impact cushioning.
The relatively soft inner layer 22 provides most of the torque absorption and impact cushioning when a workpiece is struck. In one embodiment, a plurality of rib or fin-like structures 40 are provided around the gripping structure 16 as shown in
In a preferred embodiment, the inner layer 22 is made from a non-foamed material, as is the outer layer 24. However, in another embodiment, the inner layer 22 may be a foam material.
When a user strikes a workpiece with the tool 10, the user grips the gripping structure 16 and manually swings the tool 10 to impact the striking surface 18 on the workpiece. When the impact head 12 hits the workpiece, a portion of the kinetic energy of the impact is transferred through the internal core structure 14 back to the user's hand. In an off center hit, torsional effects are increased and are transmitted to the user.
The inner layer 22 of the exterior impact-cushioning gripping structure 16 cushions the impact and increases user comfort. Due to the low Modulus of Elasticity of a low durometer TPR, the inner layer 22 allows for equivalent angular deflection of the tool internal core structure 14 without transmitting as much torque as similar materials of higher durometer, thereby “controlling” or limiting the effects of torsion resultant from off center strikes with the tool. The inner layer 22 also more effectively dampens the vibrations that occur in the internal core structure 14 following the impact of the impact head 12 on the workpiece.
In the embodiment of the hammer shown in
Applying an exterior impact-cushioning gripping structure 16 reduces the transmission of torque from the internal core structure 14 to the exterior grip 16 held by the user. This is because during an “offstrike” or some type of impact in which the hammer head hits a structure in a manner that tends to impart a generally twisting action to the core structure 14 about its longitudinal axis, the core structure 14 is permitted to twist slightly about the longitudinal axis A (as represented schematically in
As shown by comparison of
As can be appreciated form a comparison of the test results of
The following tables list the impact response dynamometer force test results conducted on six impact tools constructed in accordance with the present invention and six conventional impact tools having characteristics similar to those described above with respect to
The impact testing device incorporated a dynamometer mounting for a clamp used to hold the handle of the impact tool. The dynamometer measured the net in-plane and out of plane forces resulting from impact by an adjustable height swing arm. The impact contact point on the device was adjustable to accommodate different offset locations and impact angles. The swing arm impact tip utilized was a hard tip commonly used on impulse testing impact tools. The actual forces experienced by the dynamometer included force components acting in the direction of impact as well as force components acting in the opposite direction (due to the lever arm effect and the handle pivot point being located near the center of the dynamometer table). These forces could be resolved by a moment analysis if the location of the pivot point is known. The peak impact force could also be determined from the moment analysis if the impact force-time history is also known (measured). Additional information (impulse-momentum, etc.) could also be obtained from a calculation of the area under the force-time curves. The force measurements are in terms of peak volts as determined from the force time plots (the dynamometer sensitivity is about 20 pounds force per volt based on a static calibration of the in-plane force). The in-plane net peak force data (volts) for an offset impact location (¼″ off center; directly above the head center) is shown for two selected impact swing arm height settings (corresponding to light (force level 1) and medium (force level 2) impact).
The in-plane net peak force data for force level 1 impacts shown above is based on time domain data averaged over 4 impacts; and is considered to be more representative than the single impact time data used to determine net peak force 2 (impacts using force level 2 were conducted last and were limited to a single test per impact tool to avoid possible handle/epoxy bonding failures). The level 2 force experiments along with several auxiliary experiments provided insight into the usefulness of low level impact testing for the type impact tools (such as with hand held instrumented impulse impact tools as opposed to the swing arm impact device). The out of plane net peak force data exhibited a similar trend as the in-plane data. However, the out of plane forces are nearly an order of magnitude lower than the in-plane forces.
The results for in-plane net peak force indicate a general reduction in net peak force measured by the dynamometer for impact tools with softer “feeling” rubber handles; with impact tool “A” appearing to softer than impact tool “B.” This is generally consistent with the natural frequencies (in Hertz) for in-plane and out of plane vibration, which are also shown in the tables above for the fundamental vibration modes (in general, softer rubber would be expected to result in lower natural frequencies). The in-plane and out of plane natural frequencies were determined via a simple impulse response measurement wherein the impact tool mounted in the test fixture was impacted in the in-plane and out of plane directions and the vibration decay was observed. Small variations (±1 Hz or so) within sets of “identical” impact tools are expected due to minor variations in geometry and mounting details, however, large variations within sets (greater than 5 Hz) are indicative of significant differences between the impact tools (or the impact tool mounting details) which could be capable of significantly affecting the overall shock/ vibration performance of the impact tool. The natural frequency values, spacing between the in-plane and out of plane natural frequencies, and natural frequency vibration decay rates are governed by boundary conditions (mounting), geometry, mass, stiffness and damping properties. These factors would be expected to influence the impact response forces, the rubber handle compression and spring back characteristics, and various other aspects of the overall shock/vibration behavior of the impact tools.
While specific embodiments have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative and not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.
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