Tapered roller bearing

Description of equivalent WO 2004028369 A2

TAPERED ROLLER BEARING

The present invention relates to medical imaging arts. In particular, it relates to a rotating gantry such as those found in 3^rd and 4^th generation CT scanners, and will be described with particular reference thereto. However, the invention will also find application in conjunction with nuclear cameras and other imaging systems with rotating bearings, and is not limited to the aforementioned application. Typically, 3^rd and 4^th generation CT systems have rotating gantries and stationary gantries. The two gantries are interfaced by a bearing system that allows rotation of the first gantry relative to the second gantry.

A large ball bearing assembly, often a meter or more in diameter, has been used to provide the interface between the gantries. Large ball bearing assemblies are expensive and tend to be noisy.

In other systems, roller bearings have been used. Cylindrical rollers support the rotating gantry in both axial and radial directions. Typically, the rotating gantry has three bearing races, or tracks along which the bearings roll. A circumferential race allows the bearings to give the rotating gantry radial support (a normal force counteracting the force of gravity) while the second and third races allow bearings to give the rotating gantry lateral, that is, axial support. To prevent the rotating gantry from wobbling, the roller bearings press against the second and third races with significant opposing pressure.

While the gantry rotates with a constant angular velocity, portions of the gantry move with different radially dependent linear velocities. More specifically, portions of the second and third races more distant from the rotational axis of the gantry have a higher linear velocity than portions closer to the rotational axis. Stated differently, the linear velocity of any moving element is a function of radial position, as well as angular velocity of the gantry.

- i - This is significant to, among other things, the second and third axial support bearing races . The outer edges of these two races move faster than the inner edges of the same races. Each cylindrical roller bearing that contacts the second and third races only rotate at a single speed. Thus, slippage occurs between the bearing races and the roller bearings, causing high friction and wearing both the bearing races and the bearings prematurely. Additionally, functional speeds of the gantry are limited, in order to balance the speed of the gantry and the wear that higher speeds incur on the races and the bearings .

The present invention contemplates an improved apparatus and method, which overcomes the aforementioned limitations and others.

According to one aspect of the present invention, a diagnostic imaging device is provided. First and second gantries are interfaced by a plurality of tapered roller bearings that provide support for the second gantry as it rotates. The bearings interface bearing races on the rotating gantry.

According to another aspect of the present invention, a method of diagnostic imaging is provided. A first, rotating gantry is supported with a plurality of tapered roller bearings attached to a second gantry. The first gantry is rotated concurrently with the roller bearings, races of the gantry being in contact with the roller bearings.

One advantage of the present invention resides in increased life of the roller bearings and bearing races.

Another advantage resides in a reduced total number of roller bearings required.

Another advantage is that rotating friction of the bearing system is reduced. Another advantage is that a single size of roller bearing can be used. Another advantage resides in fewer precision machined bearing races .

Another advantage resides in a smaller rotating gantry. Another advantage resides in an integrated drive motor.

Another advantage resides in reduced cost over similar systems currently in production.

Yet another advantage resides in faster rotational speeds.

Numerous additional advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiment .

The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps . The drawings are only for the purpose of illustrating preferred embodiments and are not to be construed as limiting the invention.

FIGURE 1 is a diagrammatic illustration of a computed tomography scanner, in accordance with the present invention;

FIGURE 2 is a cross-sectional view of the rotating gantry of FIGURE 1 and tapered roller bearings, in accordance with the present invention;

FIGURE 3 is a detailed view of the roller bearings of FIGURE 2 including a drive motor;

FIGURE 4 is a geometrical representation of a bearing-race interface, in accordance with the present invention;

FIGURE 5 is an alternate two-face race embodiment of the present invention;

FIGURE 6 is an alternate three-face race embodiment of the present invention. With reference to FIGURE 1, a CT scanner 10 includes a subject couch 12 for moving a subject disposed thereon into and out of an imaging region 14. X-rays from an x-ray source 16 are shaped and collimated into a fan beam, pass through the imaging region 14 and are detected by a detector assembly 20 on the far side of the imaging region 14. In the illustrated 3^rd generation embodiment, the source 16 rotates concurrently with the detector assembly 20, always remaining 180° around the imaging region 14 from the detector assembly 20 as it rotates around an axis A. Alternately, a stationary ring of individual detectors on the stationary gantry 22 can replace the detector array 20, as in 'a 4^th generation CT scanner.

Intensities of detected x-rays are collected in a data memory 30 as a rotating gantry 24 rotates about the subject. As the data is collected, a reconstruction processor 32 applies a convolution and backprojection algorithm, or other suitable reconstruction technique, to the collected data, forming an image representation. The image representation (s) are stored in an image memory 34. A video processor 36 withdraws selected portions of the image representations and formats them for viewing on a human readable monitor 38 such as a CRT monitor, active matrix monitor, LCD display, or the like. The first, rotating gantry 24 is disposed within the second, stationary gantry 22. The x-ray source 16 and the detector array 20 are mounted on the rotating gantry 24 along with other associated electronics 44, such as power supplies, data buffers, etc.

The rotating gantry 24 is supported within the stationary gantry 22 by a plurality of tapered roller bearings

46. In the preferred embodiment, there are four sets of two bearings, making eight roller bearings total. Of course, the number of bearing pairs can be more or less, dependent upon other factors such as the weight of the gantry 24, functional speeds, and the like. Each bearing 46 rotates freely about its own bearing axle, the axle being mechanically fastened to the stationary gantry 22.

With reference to FIGURE 2, the bearings 46 interface with a conical bearing race 50. The race 50 provides surfaces angled with respect to the axis A so that the bearings 46, as shown' in FIGURE 2, provide both radial (directions perpendicular to the axis A) support and axial (directions parallel to the axis A) support.

As discussed in the background, the race 50 moves with constant angular velocity, but portions of the race 50 farther from the axis A have higher linear velocities than portions closer to the axis A while the gantry 24 is rotating.

The bearings 46 are tapered into conical shapes to compensate for the linear velocity deviation. When the gantry 24 rotates, each bearing 46 in contact with the gantry 24 also rotates. Being conical in shape, the bearings each have a wide or larger diameter end and a narrow or smaller diameter end.

Like the gantry, the surface at wide end of the bearing moves with a higher linear velocity than the surface at the narrow end. The bearings 46 are shaped with a varying diameter that is proportional to the slope and radial altitude of the race 50 such that there is no slippage between the bearings 46 and the race 50 as they all rotate.

Preferably, the bearings 46 are constructed of a metal core, preferably stainless steel, and coated with a polymeric coating, preferably polyurethane . The coating is preferably more than a surface coat, and more akin to a tire on a tricycle wheel, or the like. The coating is thick enough to provide for smooth cushioning, but thin enough that it stiffly supports the rotating gantry. Although polyurethane is preferred, other coatings that provide adequate stiffness (preventing axial and radial movement of the gantry 24) while preventing metal-to-metal contact between the race 50 and the bearings 46 are contemplated. With reference to FIGURE 3 , one of the bearings is attached to an external drive motor 52 and becomes a drive bearing 54. The drive bearing 54 is coated with a substance that can be different from the other bearings 46 for improved friction with the bearing race 50. Such a substance may be a hardened rubber or the like.

In order for the conical bearing race 50 and the tapered bearings 46 to be velocity matched, an angle of expansion that describes the growth of the diameter of a bearing 46 along the axle is found. With reference to FIGURE 4, the angle φ is found to cause velocities V₀ and V_± to match on both the race 50 and the bearings 46. First, a ratio of race 50 diameter to bearing diameter is found,

R = d_n

where D₀ is the outer diameter of the race 50 and d₀ is the outer diameter of the bearing 46 and R is the ratio of the two measurements . It follows that the angular speed of the bearing ω_t is found by

ω_t = ω_gR

where ω_g is the angular speed of the gantry in rpm. From geometry of the system it is known that

L. =

2sinor where L_c is the length of the contact surface of the roller bearing 46, and D_± is the inner diameter of the bearing race

50, and α is the angle of elevation of the bearing race 50. Solving for D_{± r}

D_i = D₀ - 2L_c shι .

In order to velocity match the contact surfaces, the velocities V₀ and V_± at the extremities of the bearing 46 and race 50 are found to match:

Z>

V_ ω_t a>„

Solving for ω_t,

ω_t c „ d„

where d₀ is the outer diameter of the bearing 46. Similarly,

where d_± is the inner diameter of the roller bearing 46 Combining the above two equations, it is found that

d, = ⁰, — D

Finally the angle φ can be found by: φ = <img file="WO2004028369A2_D0001.tif" />

The bearing axial length L_r can be found by :

L_r = L_c cos φ

but it is to be understood that the actual length of the roller bearing 46 can vary to be slightly longer or shorter, keeping the same angle φ. In an alternate embodiment, and with reference to

FIGURE 5, a race 60 is an inverted negative of the race 50.

This alternate race 60 still provides both axial and radial support for a gantry 62.

In another alternate embodiment, and with reference to FIGURE 6, a race 70 has three faces, tapered roller bearings

72 being adjacent the vertical sides of the race 70. A cylindrical (non-tapered) roller bearing 74 is adjacent the horizontal face of the race 70 since the entire face is equidistant from the axis A and thus does not display a velocity mismatch phenomenon. In this embodiment, the flat bearings 74 provide a gantry 76 with radial support, while the tapered roller bearings 72 provide the gantry 76 with axial support .