Electric Motor Mount Having Vibration Damping

The invention is related to the field of mounts for electric motors and in particular to a motor housing for a string trimmer having a motor vibration damper.

In devices using electric motors, such as string trimmers, the electric motor is supported inside a motor housing by a support structure. These support structures may take various forms as taught by the prior art. One type of support structure commonly used is a series of transverse support ribs which engage the sides of the electric motor and inhibit any transverse displacement of the electric motor relative to the housing. In many of these devices, the member driven by the electric motor is rotatively unbalanced and which upon rotation impart a vibration to the motor itself. The vibration of the motor produces a transverse force on the ends of the support ribs engaging the motor which, after a period of time, deforms the support ribs allowing the motor to vibrate independent of the motor housing. This latter condition results in a hammering of the ends of the support ribs by the motor further aggravating their deformation. This also increases the vibrational force transmitted to the motor housing. This deformation of the motor support ribs is accelerated when the motor housing is made from a structural plastic in which the support ribs are integrally molded into an external shell or housing.

In hand-held devices, such as a string trimmer, the increased vibration of the housing is transmitted to the user and produces an unacceptable sensation and, on occasion, produces a numbness of the user's limbs. This increased vibration also leads to catastrophic failure of the support ribs resulting in a significant shortening of the useful life of the device.

The invention is a vibration damper disposed between the electric motor and the motor housing which reduces the deformation of the support ribs, absorbs a significant portion of the vibrational energy imparted to the motor by the unbalanced driven device, reduces the amplitude of the vibration imparted to the motor housing and increases the operating life of the device.

The invention is a vibration damper for a small electric motor rotating an unbalanced driven member, and in particular, a vibration damper provided in the motor housing of a hand-held string trimmer. The object of the invention is to dampen the vibration produced by the driven head of a spring trimmer. The advantage of the vibration damper is that it significantly reduces the vibration of the motor housing which is transmitted to the user. Another advantage is that it reduces the forces acting on the internal ribs of the motor housing, reduces the deformation of these ribs and increases the life of the string trimmer.

A motor housing incorporating the vibration damper has at least two motor support ribs which rigidly support the electric motor therein. A pair of diametrically disposed damper chambers are provided at the end of the motor housing adjacent to the output end of the electric motor. The damper chambers are bounded between an end face member spaced a predetermined distance forward of the last motor support rib and the last motor support rib. A plurality of stiffner ribs are disposed between last motor support rid and the end face member which form a radial seat in each damper chamber. A resilient linear member is compressively disposed between the radial seats and the electric motor to absorb the vibrations imparted to the electric motor caused by the rotation of the unbalanced driven member.

Further objects and advantages of the vibration damper will become apparent from a reading of the detailed description of the invention with respect to the drawings.

FIG. 1 is an exploded view of a string trimmer motor housing embodying a vibration damper;

FIG. 2 is a cross-section of the motor housing showing the conical seat formed by the ends of the stiffner ribs; and

FIG. 3 is a cross-section showing the relationship of the stiffner ribs to the end face member and the adjacent motor support rib.

The motor housing 10 of a typical string trimmer, as shown in FIG. 1, comprises two molded plastic housing members 12 and 14 which are substantially mirror images of each. Each of the housing members has a motor mounting portion 16 and a handle mounting portion 18. The handle mounting portion 18 has an end closure member 20, a plurality of transverse handle support ribs 22 and a stop rib 24. The internal edges of the end closure member 20 and the support ribs have contoured recesses 26 in which the handle of the string trimmer is received. The size and shape of the contoured recesses 26 are selected such that the handle is rigidly secured in the handle mounting portion 18 when the molded housing members 12 and 14 are secured to each other by bolts 30 through 38. Bolt 38 passes through a first aperture 40 provided through the handle portion 18 of the housing member 16 through a second aperture 42 provided through the handle 28 and is threadably received in an internally threaded mounting post 48, provided in the handle portion 18 of the housing member 12. The bolt 38 secures the handle portions 18 of the two housing members 12 and 14 to each other and prohibits the axial displacement of the handle 28 relative to the motor housing 10.

The stop rib 24 has a arcuate recess 50 which has a diameter less than the diameter of the handle 28. The aperture formed by the actuate recess when the two housing members 12 and 14 are secured to each other permits the electrical lead 52 and 54 to pass from inside the handle 28 into the interior of the motor portion 16. During the assembly of the motor housing 10, the stop rib 24 axially positions the handle portion so that the aperture 42 in the handle is aligned with the aperture 40 and the internally threaded post 48.

Each housing portion 16 has a transverse internal rib 56, a plurality of transverse motor support ribs 58 through 62, and an end face rib 64. Each of the support ribs 58 through 62 has an arcuate relief 66 which has a diameter substantially equal to the diameter of the electric motor 68. The electric motor 68 is received in the arcuate recesses 66 and is rigidly supported in the motor housing 10 by the motor support ribs 58-62. The internal end rib 56 of each of the housing members 12 and 14 has a radially disposed key 70 which is received in a mating slot 72 provided at the internal end of the electric motor 68. The keys 70 prevent the motor 68 from rotating relative to the motor housing 10.

A pair of transverse keys 74 engage mating slots 76 provided in the housing of the motor 68. The transverse keys 74 are slidably received in the space between motor support ribs 58 and 60 and prevent the axial displacement of the motor 68 relative to the motor housing 10.

The end face member 78 is disposed at the end of the motor mounting portion 16 opposite the internal end rib 56. Each end face member 78 has a semi-circular relief 80 which when mated with a corresponding semi-circular relief 80 of the housing member 14 forms a clearance aperture in the motor housing 10 for the shaft 82 of the electric motor 68. When the motor 68 is inserted in the motor support portion 16 of the housing members 12 and 14, the forward end 84 of the motor effectively closes the aperture formed by the semi-circular reliefs 80.

As shown in FIG. 1, the housing member 12 may have a plurality of thread posts 94 which threadably receive the bolts 30 through 36 to secure the housing member 14 to the housing member 12 with the electric motor 68 and the vibration damper members 92 therein.

FIGS. 2 and 3 are partial cross-sections showing a series of parallel stiffner ribs 86 extending between the end face member 78 and the motor support rib 62. The internal ends 90 of the parallel stiffner ribs 86 form an arcuate seat for the resilient vibration damper 92. The vibration damper 92 is a resilient linear member having the general form of a rectangular parallelopiped in its relaxed or undistorted state as shown in FIG. 1 and is compressively deformed in the motor housing 10 to engage opposite side portions of the motor 86 as shown in FIG. 2. Alternatively, the vibration damper may have a circular cross-section or any other shape known in the art. Also, it is not required that the vibration damper have a linear shape, but vibration dampers having an arcuate shape are also contemplated as being within the scope of the invention.

The width of the vibration damper 92 is substantially equal to the axial spacing between the motor support rib 62 and the end face member 78 and its height is greater than the distance between the internal ends 90 of the parallel stiffner ribs 86 and the electric motor such that the vibration dampers 92 are under compression when housing members are secured to each other using bolts 30 through 36. In the assembled state, a string feeder head (not shown) is attached to the rotary output shaft 82 of the motor 68.

When first assembled, the motor support ribs 65 through 62 in each of the motor housing members 12 and 14 compressively engage the motor 68 and prohibit transverse movement of the motor 68 relative to the motor housing 10. As is known in the art, the string feeder head is rotatively unbalanced, due to the cutting string extending radially therefrom. The unbalanced string feeder head imparts a vibratory force to motor 68 through the rotary output shaft 82. This vibratory force is transmitted from the motor 68 to the motor housing through the motor support ribs 58 through 62. After a period of time, this vibrational force deforms the arcuate reliefs 66 on the motor support ribs 58 through 62 such that the motor is no longer rigidly supported in the motor housing 10. After this happens, the motor is free to vibrate relative to the housing and the amplitude of vibration of the motor increases resulting in an increase of the vibratory force imparted by the motor 68 on the motor support ribs 58 through 62. This further enlarges the arcuate reliefs of the motor support ribs and increasing the amplitude of the vibration of the motor housing 10 to the point where it becomes objectionable to the user.

The vibration dampers 92 engaging the sides of the motor adjacent to the end face members 78 resiliently absorb the vibrational force of the motor 68 significantly reducing the vibration of the motor housing 10 and the vibration transmitted to the user. The vibration dampers 92 also significantly reduce the deformation of the arcuate relief of the motor support ribs. The vibration dampers 92 not only results in a string trimmer in which the vibration transferred to the user is significantly reduced but also results in a string trimmer having an extended useful life.

Having described and illustrated a motor housing for a string trimmer having a vibration damper for reducing the vibrations transferred to the user and which also extends the operating life of the string trimmer, it is recognized that those skilled in the art may conceive improved or alternate motor housing structures having vibration dampers that are within the spirit of the invention as described herein and set forth in the appended claims.