You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND 
   There are countless applications for an apparatus that can grasp and lift a heavy or awkward workpiece. One example is a post puller that grasps an elongated object such as a fence post or a telephone pole and lifts it out of a post hole or positions it in a post hole or simply moves it about from place to place. A typical post puller consists of some type of clamp that is attached to a machine with lifting power, for example a tractor or a skid-steer loader. The clamp may be nothing more complex than a length of heavy chain that is wrapped tightly around the post, securing the post to the lifting machine. Such a clamp enables a remotely located worker such as a farmer with no power machinery other than a tractor to use the power lifting capacity of the tractor to place and remove fence posts or other awkward or heavy objects. 
   A chain used as a clamp may require a person to hold the chain securely around the post during the lifting and moving operation. If the chain is attached to a tractor with lifting capability, it may be possible for one person to simultaneously hold the chain and operate the tractor, but at best this is clumsy, and it often poses safety issues, so a second person may be needed. Lifting machines such as tractors or skid-steer loaders generally lift by pivoting about a point, and this results in the lifting motion being arcuate rather than linear. When inserting a post into, or removing it from, a deep post hole, an arcuate lifting motion can cause the post to bind against the walls of the hole, damaging the hole or the post or rendering the lifting operation impossible. 
   Accordingly, there has been a need for a lifting apparatus that can easily be attached to a lifting machine in a remote location, that can safely and conveniently be operated by a sole worker, and that can lift clumsy or heavy objects. It would be desirable for such an apparatus to lift an object through a linear rather than an arcuate range of motion. 
   SUMMARY OF THE INVENTION 
   The invention provides a lifting apparatus that attaches to a lifting machine and enables the machine to lift a heavy or awkward object under control of one person. In some aspects this lifting apparatus lifts heavy or awkward objects through a linear range of motion. 
   In some aspects the invention provides a lifting apparatus that includes a liftable frame, a first gripper slidingly carried by the frame, a second gripper carried by the frame, and an actuator that urges the first gripper slidingly toward the second gripper to grip a workpiece between the two grippers. In some aspects the lifting apparatus is adapted for attachment to a machine with lifting power such as a tractor or skid-steer loader. The actuator may be operated by hydraulic, pneumatic, electrical, or other power drawn from the machine. In some aspects the grippers are pivotingly mounted such that they pivot as the frame is lifted, thereby maintaining the workpiece in an approximately constant angular orientation despite any arcuate motion of the lifting apparatus. 
   Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  provides a perspective view of a lifting apparatus embodying the invention. 
       FIG. 2  provides an isometric view of a lifting apparatus embodying the invention, showing a workpiece positioned between the grippers. 
       FIG. 3  provides a top view of a lifting apparatus embodying the invention. 
       FIG. 4  provides a front view of a lifting apparatus embodying the invention. 
       FIG. 5  provides a side view of a lifting apparatus embodying the invention. 
       FIG. 6  provides a side view of an alternate embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   As shown in the drawings for purposes of illustration, the invention is embodied in a lifting apparatus that attaches to a lifting machine and enables the machine to lift a heavy or awkward workpiece under control of one person, in some aspects lifting the workpiece through a linear range of motion. Various ways of clamping a workpiece to a lifting machine have been used, but these have required more than one person for safe and convenient operation or have lifted the workpiece through an arcuate range of motion. 
   In the following detailed description and in the several figures of the drawings, like elements are identified with like reference numerals. 
   In some embodiments the invention provides an apparatus generally  11  for lifting a workpiece such as a fence post  13 . The apparatus includes a liftable frame  15 , a first gripper  17  slidingly carried by the frame, a second gripper  19  carried by the frame, and a hydraulic actuator  21  that urges the first gripper slidingly toward the second gripper to grip the workpiece between the two grippers whereby the workpiece is lifted when the frame is lifted. In one embodiment a first pivoting mount  23  is provided between the first gripper and the frame, and a second pivoting mount  25  is provided between the second gripper and the frame. As best shown in  FIG. 1 , the first pivoting mount  23  may comprise a sleeve  27  carried by the frame and a shaft  29  rotatably disposed in the sleeve. The second pivoting mount may be constructed in a similar fashion. 
   A pneumatic actuator, an electric motor, or some other device may be used rather than the hydraulic actuator  21 . Although a separate power supply such as a hydraulic pump, an air compressor, or the like could be used to power the actuator, the actuator can conveniently be powered directly from the lifting machine. For example, a tractor with a hydraulic system can provide the hydraulic power to operate the actuator. 
   In some embodiments the frame  15  is adapted to be lifted by a lifting element of a vehicle such as a tractor or a skid-steer loader (not shown). For example, in the embodiment illustrated, the frame  15  is constructed of a lower cross member  31 , an upper cross member  33 , a first vertical member  35  and a second vertical member  37 , the four members defining a generally rectangular shape. A first engagement member  39  is attached to the first vertical member  35  and a second engagement member  41  is attached to the second vertical member  37 . The lifting element of the vehicle engages the two engagement members to lift the frame  15 . Or the vehicle may be connected to the frame in some other manner. 
   In some embodiments the first gripper  17  is slidingly mounted to the frame  15 . This may be done by attaching the sleeve  27  to a sliding unit  45  that slides along the lower cross member  31 . 
   In the embodiment shown, the second gripper  19  is fixedly attached to the frame  15  through a mount  47 . In other embodiments the second gripper is slidingly mounted to the frame  15 , for example by means of a sliding unit similar to the sliding unit  45 . In such embodiments the actuator  21  may be connected between the two grippers rather than between one gripper and the frame, such that the actuator urges the two grippers toward each other. 
   To use the apparatus to extract a post such as the post  13  from a hole in the ground, the operator maneuvers the lifting machine so as to place the second gripper  19  against one edge of the post and a front surface  49  of the lower cross member  31  against the back of the post as best shown in  FIG. 2 . At this point the operator stops forward motion of the lifting machine. Then the operator deploys the first gripper  17  laterally to make contact with the post, pushing the post against the second gripper  19 . Pressure exerted on the post by the first gripper clamps the post against the second gripper. Once this is done the operator causes the lifting machine to lift up. This raises the frame, lifting the post out of the hole. 
   The sliding motion of the first gripper along the lower cross member  31  can be limited if desired. One way to do this is to place a pin in one of a plurality of holes  48  in the lower cross member. When the sliding unit  45  encounters a pin in one of these holes, it will stop. This is especially useful when lifting studded steel fence posts because such posts can easily be damaged if subjected to too great a gripping force. Such posts are of uniform size and can be readily grasped without damage when the travel distance to closure is limited by an appropriately placed pin. 
   An object lying on the ground can be gripped and moved in a similar manner. In this case the operator may wish to use the lifting machine to tilt the frame toward a horizontal position to more conveniently grip the object. 
   Although the invention finds an application in placing and removing fence posts, it can also be used for gripping and moving other objects such as telephone poles, pipes, logs, or other heavy and awkward objects. 
   The pivotal mounting of the two grippers minimizes any possible damage to a fence post or other object being inserted into or removed from a hole by allowing for pivoting (rotation) of the post during vertical motion. This is a useful feature of the invention because a lifting machine typically lifts through an arc rather than linearly. If the post were rigidly attached to the frame, it too would be made to travel through an arc. The pivotal mounting of the grippers enables the post to move linearly—that is, in an essentially constant vertical orientation relative to the hole—while it is being raised out of or lowered into the hole. 
   In some embodiments the grippers  17  and  19  are positioned very close to the lower cross member  31  to minimize any gap between the grippers and the cross member and thereby prevent a slender post from getting wedged in between one of the grippers and the lower cross member. 
   Various sizes and shapes of grippers may be used depending on the size, shape and weight of the object to be lifted. In the embodiment shown in the drawings, the second gripper has a round planar surface  51  with a diameter of about eight inches, slightly recessed within a lip  53 . This configuration has been found to work well with various kinds of posts. A pipe-vice gripper having vertical grooves could be used for gripping iron pipes. Other sizes and shapes of grippers could also be used as desired. 
   By means of a lifter embodying the invention, a single worker can traverse an existing fence row, removing the posts one by one and dropping each on the ground near the hole. Later the posts can be retrieved, or if the fence is being replaced, the posts can be repositioned in their holes. 
   Although several embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The invention is limited only by the claims.

Summary:
Apparatus for gripping and lifting objects such as fence posts. A frame carries a pair of grippers that are slidingly movable along the frame to grasp a post or other object. The grippers may pivot so that the object remains in essentially constant vertical orientation despite arcuate motion of a tractor or other lifting machine by which the frame is lifted. The grippers may be actuated by hydraulic or other power provided by the lifting machine.