while the present invention will be fully described hereinafter with reference to the accompanying drawings , in which particular embodiments are shown , it is to be understood at the outset that persons skilled in the art may modify the embodiments disclosed herein while still achieving the desired result . accordingly , the description that follows is to be understood as a broad informative disclosure directed to persons skilled in the appropriate art and not as limitations of the present disclosure . fig1 and 2 show a wheel assembly 10 as part of a caster 12 as often set between a pair of legs 14 and 16 connected by a base 17 of an inverted u - shaped frame 18 of which legs 14 and 16 are a part thereof . preferably , the legs 14 and 16 extend away from the base 17 parallel to each other . the legs 14 and 16 preferably taper in width as they extend away from the base 17 . the ends 19 and 21 of legs 14 and 16 may be semi - circular , forming substantially u - shaped end portions 23 and 25 . each end portion 23 and 25 has a hole 27 and 28 respectively concentric with the semi - circular ends 19 and 21 . the base 17 of the frame 18 can be attached by a stem 29 extending therefrom to a shopping cart , furniture moving cart , trolley , baby walker , wheelchair or other objects with a potential for wheels . the stem 29 can also be threaded . the base 17 may be a plate for various types of attachment to objects . the legs 14 and 16 straddle the wheel assembly 10 and a bolt 31 may extend through a pair of axially aligned holes 27 and 28 in the legs 14 and 16 and a hollow interior 33 of an axle 34 to render the wheel assembly 10 relatively rotatably supported on the frame 18 . preferably , the base 17 includes a swivel to permit free turning of the caster 12 relative to the cart or similar other objects . as such , the wheel 36 may rotate in one direction . the particular preferred frame is not meant to limit the invention , and “ frame ” may include a structure designed to hold the wheel 36 . wheel 36 may have a hub 37 with a central opening 38 . a bearing assembly 40 is preferably mounted in the opening 38 , and the bearing assembly 40 is preferably dual sealed , as shown in fig3 . fig1 shows a common 6002zz precision ball bearing . regarding bearing assemblies , roller bearings carry heavier loads while ball bearings roll more easily but carry lesser loads . the wheel 36 can be rotatably supported on a cylindrical axle 34 by means of the bearing assembly 40 . as such , the axle 34 can be part of the bearing assembly 40 . the radial periphery 42 of the hub 37 preferably defines a tread mounting surface . the outer periphery 42 of the hub 37 may have synthetic resin material tread 44 secured thereto . a moldable synthetic resin tread material , particularly polyurethane , may be used as the tread 44 . the thread guards 46 and 48 each have a circular hole 50 and 52 respectively . thread guards 46 and 48 are on opposite side of the wheel 36 . as shown , the small thread guard 46 may be made from a compressible or flexible nylon or similar wear - resistant material . the thread guard 48 forming a housing 54 is preferably made from a rigid nylon or similar wear - resistant material . holes 27 and 28 of the legs 14 and 16 are coaxially aligned with the holes 50 and 52 of the thread guards 46 and 48 . the thread guards 46 and 48 are fixed against rotation relative to the frame 18 . a bolt 31 may be inserted through a hollow interior 33 of the axle 34 and the sets of coaxially aligned holes 27 and 28 of the legs 14 and 16 are coaxially aligned with the holes 50 and 52 of the thread guards 46 and 48 , respectively , so that the axle 34 is mounted upon the legs 14 and 16 of the frame 18 . a nut 56 can be screwed on the threaded end of the bolt 31 in order to prevent removal of the bolt 31 from the frame 18 . the thread guard 48 forming the housing 54 has an outer side 58 as shown in fig6 and an inner side 60 as shown in fig4 . the outer side 58 preferably forms a circle extending to the outer periphery 42 of the hub 37 to minimize contamination of the wheel 36 . the outer side 58 may have stops 62 and 64 on each side of end 19 of leg 14 . the inner side 60 may have a recessed area 66 with a fixed pocket 68 , which preferably remains at the top of thread guard 48 . other features of the thread guard 48 could be a semi - circular groove 70 on the inner side 60 as shown in fig4 and a dampening material 72 as shown in fig1 and 12 . as an option , a soft material can be adhered or molded to the inside of the thread guard 48 to absorb the clack of the ball 74 as the wheel 36 rotates . preferably , this soft material is an elastomer that dampens sound and eliminates the clack and rattle of the moving ball 74 . the groove 70 may extend around a portion of the thread guard 48 to form a wall as the portion of the perimeter of the recessed area 66 that does not include the fixed pocket 68 . as shown in fig1 , the thread guard 48 can form a housing 54 without a groove and smaller than outer periphery 42 of the hub 37 . the fixed pocket 68 has rounded ends 76 and 78 forming corners 80 and 82 respectively . the area of the fixed pocket 68 between rounded ends 76 and 78 is sufficient to hold ball 74 , but the pocket 68 is preferably shallower in the radial depth from hole 52 than the diameter of the ball 74 . thus , the ball 74 extends slightly from the pocket 68 when the ball 74 is in the pocket 68 . the fixed pocket 68 is preferably about 80 - 90 degrees ( shown at 87 degrees in the drawings ) of the perimeter of the recessed area 66 . the fixed pocket 68 is preferably at the top of the recessed area 66 so gravity will allow the ball 74 to drop from the fixed pocket 68 under the appropriate circumstances . also , in operation , the ball 74 can be forced up against gravity ( flung ) by centrifugal force into the fixed pocket 68 . in the wheel assembly 10 , a rotating insert 84 can be attached to the hub 37 or can be part of the hub 37 . the rotating insert 84 is preferably located inside the recessed area 66 of the thread guard 48 . the rotating insert 84 has a pocket 86 . the insert pocket 86 is of sufficient size to hold the ball 74 , and the insert pocket 86 may be the same depth as the diameter of the ball 74 or preferably slightly deeper . the insert pocket 86 has two edges 88 and 90 . leading edge 88 is preferably somewhat rounded with a slope into the pocket 86 . the trailing edge 90 may form a lip so the pocket 86 forms a cup with a partial circumference similar to the ball 74 . the trailing edge 90 forming a lip is preferred for a wheel 36 designed to rotate in one direction , such as for swivel casters . edges 88 and 90 can both be rounded with a slope into the pocket 86 for wheel designed to rotate in both directions . the outer radial surface of the insert 84 can be called the brake ring surface 92 , which is adapted to work with the ball 74 for braking action . the brake ring surface 92 extends from edge 88 to edge 90 around the portion of the insert 84 not including the pocket 86 . the ball 74 can be trapped and pinched between the thread guard 48 and a brake ring surface 92 of preferably soft polyurethane , which is capable of deflecting with the ball 74 , creating drag , friction , or resistance , and then springing back into its original shape . more or less drag , friction , or resistance ( braking force ) can be generated by varying the interference fit of the ball 74 with the brake ring surface 92 and the thread guard 48 . the brake ring surface 92 can include constant surface of the insert 84 , an inserted semi - circular band , or similar circular objects with an aperture in the center . an optional feature of adding serrations 94 to the brake ring surface 92 can produce a pulsing effect to help identify when the brake is engaged . the brake ring surface 92 with optional serrations 94 could also be made of compressible , abrasion resistant polyurethane . ball 74 can be flung by centrifugal force from insert pocket 86 to fixed pocket 68 in association with the braking force . the ball 74 remains internal to the wheel assembly 10 between thread guard 48 and insert 84 . for the shopping cart embodiment , a ⅜ inch steel ball is disclosed , but the ball 74 could also include a variety of friction and anti - friction balls , including different dimensions , sizes , materials , and weights . the ball 74 optionally may be lightly lubricated with grease or silicone to diminish built up frictional heat . fig1 - 20 shows how the braking action works when the speed of the wheel 36 exceeds a predetermined speed , creating enough centrifugal force to fling the ball 74 out of a pocket 86 of the rotating insert 84 into a fixed pocket 68 located in the thread guard 48 . fig1 shows the wheel 36 at rest or at travel slower than the predetermined speed . the ball 74 rests in the pocket 86 of the insert 84 . fig1 shows the ball 74 remaining in the pocket 86 at rest or at travel slower than the predetermined speed . in fig1 , the ball 74 approaches the fixed pocket 68 . in fig1 , the ball 74 is centrifugally flung out of the pocket 86 of the insert 84 into the fixed pocket 68 of the thread guard 48 , such as when the wheel 36 is moving faster than the predetermined speed . in fig1 , the ball 74 is then trapped and pinched between the thread guard 48 and a brake ring surface 92 of the insert 84 . the ball 74 is shown in the rounded end 78 . as shown in fig1 , the wheel 36 will continue to roll , but the drag , friction , or resistance of the trapped ball 74 will slow the wheel 36 as the ball 74 is pushed over the brake ring surface 92 — at least until the ball 74 reaches the pocket 86 of the insert 84 as shown in fig1 . if the wheel 36 is still rolling faster than the predetermined speed , the ball 74 will not drop back into the pocket 86 of the insert 84 , thereby starting another rotation of the braking action . when the wheel 36 is traveling slower than the predetermined speed , the ball 74 will drop back into the normal travel ball position in the pocket 86 of the insert 84 as shown in fig2 . although preferred embodiments of the disclosure are illustrated and described in connection with particular features , it can be adapted for use with a wide variety of wheels . other embodiments and equivalent assemblies , brakes , balls , and wheels are envisioned within the scope of the claims . various features of the disclosure have been particularly shown and described in connection with illustrated embodiments . however , it must be understood that the particular embodiments merely illustrate and that the invention is to be given its fullest interpretation within the terms of the claims .