This invention relates generally to vibratory screening machines and more particularly concerns shipping straps used to protect the isolator mechanisms of a vibratory screening machine during transport of the machine from site to site.
Known isolator mechanisms include one or more coil springs or other types of resiliently expanding and contracting components, such as rubber-based belts, in lieu of the springs. The springs are commonly positioned at or near each of the four corners of the machine and suspend or support the basket of the machine from or on the machine base frame. Thus, the isolator mechanisms serve as pivoting linkages between the basket and frame. A typical known isolator mechanism is hereinafter described in detail.
Whatever the type of vibratory screening machine involved, its isolator mechanism and mass configuration will have a low resonance frequency. Unless the basket is locked down, the force fluctuations encountered during machine transport are close to its resonance point. These fluctuations often cause the basket to be displaced from the base frame to such an extent that the isolator mechanism will permanently stretch, the isolator mechanism “spring rate” may change, the basket may hang unevenly and, ultimately, the isolator mechanism will fail as its elastic limit is exceeded. As these deficiencies progress, the result will be increasing machine inefficiency and possibly total inoperability of the machine. The replacement of a defective isolator mechanism, assuming a replacement is on-hand, will typically require a half day of machine/drilling rig down time at a loss rate of likely more than $8,000 per day.
The known solution to these problems requires the use of a rigid shipping strap to “lock down” the isolator mechanism. A typical known shipping strap is hereinafter described in detail. The strap prevents any expansion or compression of the springs or equivalents during transport. This solution introduces problems of its own.
Prior to transport, if the springs are stretched beyond their normal load length, levers must be used to raise or lower the basket level to bring the springs to their normal load length so that the rigid strap can be aligned with the isolator mechanism. Once the springs are at their normal load length, the rigid strap can be installed on the isolator mechanism, but a separate tool is required to secure the strap and lock the mechanism down. In the locked-down status, the springs cannot expand at all and remain at their normal load length throughout transport. However, at the delivery site, a tool is again required to remove the strap from the mechanism.
Because of the owner's desire to achieve maximum the use of an expensive screening machine, there is generally a sense of urgency felt by rig hands to speed up the installation and removal of the machine from site to site. Rig downs are normally hectic and the tasks of installing and removing shipping straps are generally considered by rig hands to be a nuisance. Many rig hands simply do not want to take the time to perform the necessary tasks, especially when levering the basket into alignment with the straps is one of the necessary strap installation steps. If straps are not installed, they don't have to be removed. Since the tasks require separate tools and parts, for example a wrench and nuts, if the tools or parts are “lost” or “misplaced,” the shipping straps cannot be installed or removed and the shipping strap nuisance is thus avoided. But, eventually, when machines are moved without shipping straps, isolator mechanisms are stretched, shaker performance is poor and, ultimately, the isolator mechanisms fail totally and the machines will be inoperable until they are replaced.
It is, therefore, an object of this invention to provide a shipping strap assembly which eliminates the need of levers to bring isolator mechanisms to their normal loaded length before installation of the shipping strap. It is a further object of this invention to provide a shipping strap assembly which eliminates the need for tools to install or remove the shipping strap on or from an isolator mechanism. It is another object of this invention to provide a shipping strap assembly which eliminates the necessity for putting a machine basket in a locked-down condition for transport. Still another object of this invention is to provide a shipping strap assembly which eliminates the need for ever removing a shipping strap or installation part from the machine. It is also an object of this invention to provide a shipping strap assembly which reduces and simplifies the tasks involved in protecting an isolator mechanism from damage due to stretching. And it is an object of this invention to provide a shipping strap assembly which reduces the likelihood that machines will be transported without shipping straps.