1270619 九、發明說明： 【發明所屬之技術領域】 ^ 本發明是關於一種腳踏車鏈輪總成，尤其是關於一種包 含有重量輕的鏈輪載件及具有階狀鉚接孔而用以容納鉚接 軸的複數個鏈輪，一個鉚接頭及鉚接工具的鏈輪總成。 【先前技術】 通常，具有變速齒輪系統的腳踏車的鏈輪總成，包含有 、 多數個鏈輪，其等是被鏈輪載件所支持且具有遞減的直徑而 ^ 同心。例如，DE 199 37 2 1 2 A1中揭示有一種鏈輪總成，其 包含有複數個鏈輪固定到一個階狀的四臂鏈輪載體件。最大 的鏈輪支持著鏈輪載件，並且每一階容納一個鏈輪。最大鏈 輪及鏈輪載件是不轉動地被安裝到輪轂上，以用於將其它鏈 輪之扭矩轉移到輪轂上。其它鏈輪是由鉚釘而被固定到鏈輪 載件或最大鏈輪上。 另一種鏈輪總成被揭示於實用新型DE 296 23 67 1 U1 中，其包含有兩個鏈輪副總成。每個副總成包含有一個轂部 及徑向地從轂部往外延伸之載臂。兩個鏈輪使用同樣的鉚 釘，在載臂之每一面上一個而各被固定到每個副總成上。兩 個鏈輪上之齒數之間的差很小。隔件被使用單獨地將許多其 它鏈輪固定到腳踏車輪轂上。 由於希望有大數目的鏈輪，導致具有小差異之鏈輪外徑 的靠近之齒輪比間距，並且希望具有輕量的構造，因而許多 缺點與上述鏈輪總成相關。在鏈輪載件上單獨鏈輪之緊軸向 配置之一個缺點，爲突出的鉚釘頭突入鏈輪之間的空間中且 1270619 與鏈條產生干涉。另一個問題爲在鉚接過程中由鉚合工具而 傷害到鏈輪表面。故，需要及多數個鏈輪在鏈輪載件上之有 技巧性的配置及固定。 【發明內容】 本發明提供一種腳踏車的鉚接之鏈輪總成，其包含有一 個鏈輪載件及由鉚釘而固定在鏈輪載件上之複數個鏈輪。鉚 釘具有一個軸及第1及第2鉚釘頭。鏈輪包含有複數個鉚釘 孔用以容納鉚釘。鉚釘孔包含有··一個軸部，用以容納鉚釘 軸；一個埋頭孔部，用以容納第2鉚釘頭；及一個間隙部， 用以容納鉚接工具。鉚釘孔之軸部具有第1直徑，其是等於 或大於鉚釘軸的直徑。鉚釘孔之埋頭孔部具有第2直徑，其 是等於或大於鉚釘頭的直徑。第2直徑是比第1直徑更大。 鉚釘孔之間隙部具有第3直徑，其可提供間隙給鉚接工具。 第3直徑是比第2直徑更大。 鏈輪載件包含一個轂部及徑向地從轂部往外延伸之複 數個載臂。轂部包含複數個鍵部，其具有不同寬度而可與輪 轂上的對應之鍵部啣接。載臂之前面包含有階部用以容納鏈 輪，並且載臂之一個後面實質上爲平坦。載臂亦包含有與每 個階部相連的鉚釘孔，用以容納鉚釘之軸。爲了減少鏈輪載 件之重量，轂部之壁厚被減少，並且載臂到轂部之銜接變化 爲平順。又，載臂在中央區域的圓周寬度被狹窄化，並且朝 向載臂之徑向外端再度變成較寬。雖然載臂具有狹窄的中央 區域，但是載件之強度或穩定度不受影響，因爲階部使載臂 的軸向厚度徑向往內地朝轂部增加。載臂之一個端部區域的 1270619 軸向厚度是由外鏈輪之間隔所決定。 最大及第2大鏈輪是在各載臂使用單一鉚釘而側向地各 被固定到鏈輪載件之後及前面。其餘的鏈輪被固定到載臂之 前面。因爲必須固定到載件的鏈輪之大數目，故必須減少鏈 輪之軸向空間及相鄰鏈輪之間的空隙。 鏈輪被固定到載件，是首先將鉚釘之軸從載件之後面嵌 入載臂之鉚釘孔中。然後，從鏈輪延伸的軸之端部及載件之 前面，是由鉚接工具所鉚接，以形成第2鉚釘頭。在鉚接作 業中，鉚釘軸在鏈輪之鉚釘孔中膨脹，而且接觸載臂中鉚釘 孔之一部分。此作業確保鉚釘被鎖入鏈輪及載臂上之鉚釘孔 中的適當處所，因而鏈輪是以不可動的零間隙而被固定到載 臂。 在本發明之另一實施例中，一個在安裝前沒有鉚釘頭之 銷型鉚釘可被使用來取代鉚釘而將鏈輪固定到載件。在將銷 插入鏈輪及載臂二者之鉚釘孔中之後，將銷型鉚釘之兩端鉚 接而形成第1及第2鉚釘頭。在鉚接作業中，鉚釘軸變成被 鎖入鏈輪之鉚釘孔中，及載臂之鉚釘孔的各相鄰孔區域中。 本發明之一個目的在提供一種鏈輪總成，其不僅爲輕 量，而且對扭矩亦很具韌性，並且在生產及安裝特別是對鉚 接作業上均可最適化。這些目的可由鏈輪載件之臂的輕量構 造及用以容納鉚釘頭之鏈輪的鉚釘孔之階狀構造而達成。 本發明之一個目的，是將鉚釘頭埋入孔中以避免鉚釘頭 突出於軸向鏈輪外觀之外，並且由提供一個間隙延伸於鉚釘 之鉚釘頭周圍而避免鏈輪表面被鉚接工具所破壞。 1270619 以此鉚接的鏈輪總成及以另外較小的鏈輪時，較小的鏈 輪由於其小的齒數，並不配置在鏈輪載件而是配置緊鄰著鏈 輪載件，因而可組合成一種鏈輪總成，其具有很小的軸向厚 度，並且與上述專利文件中說明的方法者比較時，較輕且更 經濟。 本發明的這些及其它特徵及優點，將可由本發明某些實 施例的下列說明，參考附圖而更完全了解。 【實施方式】 第1 -4圖顯示本發明一個實施例之鉚接之鏈輪總成1。 鏈輪總成1 一般包含有複數個鏈輪2a-2f，在此實施例中爲6 個，其是由鉚釘7而固定到鏈輪載件3。鏈輪載件3包含有 一個轂部4及及徑向地從轂部4往外延伸之複數個載臂6。 轂部4包含有許多不同寬度的內鍵5，可與輪轂（未圖示）上 的對應之鍵部啣接，用以轉移扭矩。爲了減少載件之重量， 轂部4具有大的半徑，並且從轂部4到載臂6之銜接變化爲 平順。又，轂部4之壁厚及載臂6之厚度被減少。 見第2圖，載臂6包含有階部8位於鏈輪載件3之前面 用以容納鏈輪2b-2e。載臂6之一個後面實質上爲平坦。爲 了減少重量，載臂6在中央區域的圓周寬度爲狹窄，其並不 減少載件6之強度或穩定度，因爲由於載件6之前面的階部 8使載臂6的軸向厚度朝轂部4增加。載臂6之一個端部區 域的軸向厚度是由外鏈輪之間隔所決定。載臂6亦包含有鉚 釘孔24，在此實施例中每一個載臂6上有5個鉚釘孔，用以 容納鉚釘7。鉚釘7包含有一個鉚釘軸1 7及第1鉚釘頭1 6。 1270619 鉚釘7之鉚釘軸17從載臂6之後面被插入鉚釘孔24中，並 且被鉚接在鏈輪2之前面，以形成第2鉚釘頭1 8。最大鏈輪 2f及第2大鏈輪2e是使用一個鉚釘7而被固定到每個載臂 6。最大鏈輪2f被固定到載臂6的後面，並且第2大鏈輪2e 被固定到載臂6之前面。 見第3圖，鏈輪2a-2f包含有複數個安裝托架10,在此 實施例中有5個安裝托架，其是徑向地往內延伸。爲了減少 重量，除了包含有用以改善鏈輪2a-2f之間的鏈條之移位用 的側向切口 12的齒之外，鏈輪2a-2f包含有孔20位於每一 個齒下方。亦是爲了減少重量，而減少徑向地往內朝向轂部 延伸的鏈輪材料之量。鉚釘孔1 3是位於安裝托架1 0上。鉚 釘孔1 3包含有用以容納鉚釘7之鉚釘軸1 7的軸部26，一個 埋頭孔部1 1，用以容納第2鉚釘頭1 8 ;及一個間隙部14， 用以容納鉚接工具（未圖示）。鉚釘孔1 3之軸部26具有第1 直徑3 6，其是等於或大於鉚釘7之未膨脹軸1 7的直徑。鉚 釘孔1 3之埋頭孔部1 1具有第2直徑3 8，其是等於或大於第 2鉚釘頭1 8的直徑。鉚釘孔1 3之間隙部14具有第3直徑 1 5，其是被構成可提供間歇性給鉚接工具，並且具有比鉚接 工具之一個端部更大的直徑。第2直徑3 8是比第1直徑3 6 更大。第3直徑15是比第2直徑3 8更大。或者，埋頭孔部 之第2直徑可爲如間隙部之第3直徑一樣大，但是，這可能 會損害到鏈輪的強度，鏈輪已經在第2鉚釘頭的區域中具有 很開放的構造。 第4圖顯示三個最大鏈輪2d-2 f被固定到載臂6中之一 1270619 . 個。單一鉚釘7被用於將最大鏈輪2f固定到載臂6之後面， 並且將第2大鏈輪2e固定到載臂6之前面。第3大鏈輪2e 及其它鏈輪2b-2d被各被鉚接到載臂6之階部8之端面。鏈 輪2a被鉚接到載臂6之前面。爲了將鏈輪2安裝到載臂6 上，鉚釘7之鉚釘軸1 7從載臂6之後面被插入鉚釘孔24中。 第1鉚釘頭1 6局部地被埋入到載臂6後面的鉚釘孔24之埋 頭孔部40中，造成鉚釘頭1 6僅小部分延伸超過載臂6後面 • 的實質爲平坦表面。第2鉚釘頭1 8由鉚接工具而被形成在 ^ 延伸通過載臂6之前面的鉚釘軸1 7之一端。在鉚接作業時， 鉚釘軸17膨脹且壓迫鏈輪2之鉚釘孔1 3，並且壓抵載臂6 之鉚釘孔24的一部分。此不僅在鉚釘7與鏈輪2之間，而 且在鉚釘7與鏈輪載件3之間形成正面連接。此可使鏈輪2 不可動且對準地固定到鏈輪載件3。第2鉚釘頭1 8局部地被 容納於鉚釘孔1 3之埋頭孔部1 1中，且並不延伸超過鏈輪載 件3之前面。鉚釘孔1 3之間隙部1 4可防止鉚接工具在鉚接 作業中直接撞擊鏈輪2。 ® 第5圖是本發明之另一個實施例，相似於第1 -4圖的實 施例，除了鉚釘7被銷型鉚釘19所取代之外。鏈輪載件3 之後面的第1鉚釘頭30及鏈輪載件3之前面的第2鉚釘頭 32，是以鉚接工具而形成。在鉚接作業時，銷型鉚釘1 9之 鉚釘軸34膨脹且壓迫鏈輪2之鉚釘孔1 3，並且壓抵載臂6 之鉚釘孔24的一部分，不僅在鉚釘1 9與鏈輪2e及2f之間， 而且在鉚釘1 9與載臂6之間形成正面連接。此可使最大鏈 輪2f及第2大鏈輪2e不可動且對齊地固定到鏈輪載件3。 -10- 1270619 ; 第2鉚釘頭32局部地被容納於鉚釘孔1 3之埋頭孔部1 1中， 且並不延伸超過鏈輪2之前面。間隙部1 4防止鉚接工具在 鉚接作業中直接撞擊鏈輪2。 如這些圖所顯示，鉚接工具對鏈輪之傷害是由鉚釘頭周 圍的間隙所防止；並且鏈輪總成之重量是由鏈輪之開放構 造、薄的轂部壁厚、及載臂的窄化所減輕。 雖然本發明已參照許多實施例而說明，但是須了解，在 • 不違離所敘述本發明觀念的主旨及範圍之內可作許多改 變。從而，本發明不欲被已揭示的實施例所限定，而是其包 含由下列申請專利範圍之文字所容許之所有範圍。 【圖式簡單說明】 第1圖是本發明之一個實施例的鏈輪總成之立體正面 圖； 第2圖是第1圖之鏈輪總成之立體後面圖； 第3圖是第1圖之鏈輪總成之立體正面圖； 第4圖是第1圖之鏈輪總成的局部橫剖面圖，特別顯示 ^ 將鏈輪安裝到一個鏈輪載件之鉚釘；及 第5圖是本發明之另一個實施例的鏈輪總成之局部橫剖 面圖。 【元件符號說明】 1...鏈輪總成 2a-2f…鏈輪 3 ...鏈輪載件 4 ...轂部 -11- 1270619 . 5 ...內鍵 _ 6…載臂 7.. .鉚釘 8…階部 10.. .安裝托架 11.. .埋頭孔部 12···切口 • 13…鉚釘孔 ® 14...間隙部 14.. .鉚釘孔之間隙部 15.. .第3直徑 16…第1鉚釘頭 17…鉚釘軸 18…第2鉚釘頭 19…銷型鉚釘 20…鏈輪之孔 β 22…鏈輪之齒 24.. .鉚釘孔 26.. .鏈輪鉚釘孔之軸部 3 0…第1娜釘頭 3 2...第2鉚釘頭 34.. .銷型鉚釘之鉚釘軸 36.. .第1直徑 38.. .第2直徑 -12 1270619 40...載臂鉚釘孔之埋頭孔部1270619 IX. Description of the invention: [Technical field to which the invention pertains] The present invention relates to a bicycle sprocket assembly, and more particularly to a sprocket carrier having a light weight and a stepped rivet hole for receiving a rivet shaft A plurality of sprockets, a rivet joint and a sprocket assembly for riveting tools. [Prior Art] Generally, a sprocket assembly of a bicycle having a transmission gear system includes a plurality of sprocket wheels supported by a sprocket carrier and having a decreasing diameter and being concentric. A sprocket assembly is disclosed, for example, in DE 199 37 2 1 2 A1, which comprises a four-arm sprocket carrier member secured to a step by a plurality of sprocket wheels. The largest sprocket supports the sprocket carrier and accommodates one sprocket per step. The largest sprocket and sprocket carrier are mounted to the hub in a non-rotating manner for transferring torque from the other sprocket to the hub. The other sprocket is fixed to the sprocket carrier or the largest sprocket by rivets. Another sprocket assembly is disclosed in the utility model DE 296 23 67 1 U1, which comprises two sprocket sub-assemblies. Each of the sub-assemblies includes a hub and a carrier arm extending radially outwardly from the hub. The two sprocket wheels use the same rivet and are attached to each of the sub-assemblies on each side of the carrier arm. The difference between the number of teeth on the two sprockets is small. The spacers are used to individually secure a plurality of other sprockets to the pedal wheel hub. Since it is desirable to have a large number of sprockets, resulting in a close gear ratio of the outer diameter of the sprocket with a small difference, and it is desirable to have a lightweight construction, many disadvantages are associated with the sprocket assembly described above. One disadvantage of the tight axial arrangement of the individual sprockets on the sprocket carrier is that the protruding rivet head projects into the space between the sprockets and 1270619 interferes with the chain. Another problem is that the sprocket surface is damaged by the riveting tool during the riveting process. Therefore, it is necessary to have a skillful configuration and fixing of a plurality of sprocket wheels on the sprocket carrier. SUMMARY OF THE INVENTION The present invention provides a riveted sprocket assembly for a bicycle that includes a sprocket carrier and a plurality of sprockets that are secured to the sprocket carrier by rivets. The rivet has a shaft and first and second rivet heads. The sprocket includes a plurality of rivet holes for receiving the rivets. The rivet hole includes a shaft portion for receiving the rivet shaft, a counterbore portion for accommodating the second rivet head, and a gap portion for receiving the rivet tool. The shaft portion of the rivet hole has a first diameter which is equal to or larger than the diameter of the rivet shaft. The counterbore portion of the rivet hole has a second diameter which is equal to or larger than the diameter of the rivet head. The second diameter is larger than the first diameter. The gap portion of the rivet hole has a third diameter that provides clearance to the riveting tool. The third diameter is larger than the second diameter. The sprocket carrier includes a hub and a plurality of arms extending radially outward from the hub. The hub includes a plurality of key portions having different widths for engaging with corresponding key portions on the hub. Before the carrier arm, the bread contains a step for accommodating the sprocket, and one of the rear faces of the carrier is substantially flat. The carrier arm also includes rivet holes associated with each of the steps for receiving the shaft of the rivet. In order to reduce the weight of the sprocket carrier, the wall thickness of the hub is reduced and the engagement of the carrier arm to the hub changes smoothly. Further, the circumferential width of the carrier arm in the central region is narrowed, and becomes wider toward the radially outer end of the carrier arm. Although the carrier arm has a narrow central region, the strength or stability of the carrier is unaffected because the step increases the axial thickness of the carrier arm radially inward toward the hub. The axial thickness of 1270619 in one end region of the carrier arm is determined by the spacing of the outer sprocket. The largest and second largest sprocket are fixed to the front and rear of the sprocket carrier with a single rivet on each carrier arm. The remaining sprocket is fixed to the front of the carrier arm. Because of the large number of sprockets that must be secured to the carrier, the axial space of the sprocket and the gap between adjacent sprockets must be reduced. The sprocket is fixed to the carrier by first inserting the rivet shaft from the rear face of the carrier into the rivet hole of the carrier. Then, the end portion of the shaft extending from the sprocket and the front surface of the carrier are riveted by a riveting tool to form a second rivet head. In the riveting operation, the rivet shaft expands in the rivet holes of the sprocket and contacts a portion of the rivet holes in the carrier arm. This operation ensures that the rivet is locked into the sprocket and the appropriate location in the rivet hole on the carrier arm so that the sprocket is secured to the carrier arm with a non-movable zero clearance. In another embodiment of the invention, a pin-type rivet having no rivet head prior to installation can be used in place of the rivet to secure the sprocket to the carrier. After the pin is inserted into the rivet holes of both the sprocket and the carrier arm, both ends of the pin rivet are riveted to form the first and second rivet heads. In the riveting operation, the rivet shaft becomes locked into the rivet hole of the sprocket and in the adjacent hole area of the rivet hole of the carrier arm. SUMMARY OF THE INVENTION One object of the present invention is to provide a sprocket assembly that is not only lightweight but also tough to torque and that is optimized for both production and installation, particularly for riveting operations. These objects can be achieved by the lightweight construction of the arms of the sprocket carrier and the stepped configuration of the rivet holes for receiving the sprocket of the rivet head. It is an object of the present invention to embed a rivet head in a hole to prevent the rivet head from protruding beyond the appearance of the axial sprocket and to provide a gap extending around the rivet head of the rivet to prevent the sprocket surface from being damaged by the riveting tool . 1270619 When the sprocket assembly is riveted and the other sprocket is smaller, the smaller sprocket is not disposed in the sprocket carrier but is disposed adjacent to the sprocket carrier due to its small number of teeth. A sprocket assembly is synthesized which has a small axial thickness and is lighter and more economical when compared to the methods described in the above patent documents. These and other features and advantages of the present invention will be more fully understood from the following description of the appended claims. [Embodiment] Figs. 1 to 4 show a sprocket sprocket assembly 1 according to an embodiment of the present invention. The sprocket assembly 1 generally includes a plurality of sprockets 2a-2f, in this embodiment six, which are secured to the sprocket carrier 3 by rivets 7. The sprocket carrier 3 includes a hub portion 4 and a plurality of carrier arms 6 extending radially outwardly from the hub portion 4. The hub 4 includes a plurality of internal keys 5 of different widths that are engageable with corresponding key portions on a hub (not shown) for transferring torque. In order to reduce the weight of the carrier, the hub 4 has a large radius and the engagement change from the hub 4 to the carrier 6 is smooth. Further, the wall thickness of the hub portion 4 and the thickness of the carrier arm 6 are reduced. Referring to Fig. 2, the carrier arm 6 includes a step 8 located in front of the sprocket carrier 3 for receiving the sprocket 2b-2e. One of the rear faces of the carrier arm 6 is substantially flat. In order to reduce the weight, the circumferential width of the carrier arm 6 in the central region is narrow, which does not reduce the strength or stability of the carrier member 6, since the axial thickness of the carrier arm 6 faces the hub due to the step 8 of the front face of the carrier member 6. Department 4 increased. The axial thickness of one end region of the carrier arm 6 is determined by the spacing of the outer sprocket. The carrier arm 6 also includes rivet holes 24, in this embodiment each of the carrier arms 6 has five rivet holes for receiving the rivets 7. The rivet 7 includes a rivet shaft 17 and a first rivet head 16. 1270619 The rivet shaft 17 of the rivet 7 is inserted into the rivet hole 24 from the rear surface of the carrier arm 6, and is riveted to the front surface of the sprocket 2 to form the second rivet head 18. The largest sprocket 2f and the second large sprocket 2e are fixed to each of the carriers 6 using a rivet 7. The largest sprocket 2f is fixed to the rear of the carrier arm 6, and the second large sprocket 2e is fixed to the front face of the carrier arm 6. Referring to Figure 3, the sprocket 2a-2f includes a plurality of mounting brackets 10, in this embodiment there are five mounting brackets that extend radially inwardly. In order to reduce the weight, the sprocket 2a-2f includes a hole 20 below each tooth, in addition to the teeth including the side slits 12 for improving the displacement of the chain between the sprockets 2a-2f. Also to reduce weight, the amount of sprocket material extending radially inward toward the hub is reduced. The rivet holes 13 are located on the mounting bracket 110. The rivet hole 13 includes a shaft portion 26 for receiving the rivet shaft 17 of the rivet 7, a counterbore portion 1 1 for receiving the second rivet head 18; and a gap portion 14 for receiving the riveting tool (not Graphic). The shaft portion 26 of the rivet hole 13 has a first diameter 3 6, which is equal to or larger than the diameter of the unexpanded shaft 17 of the rivet 7. The counterbore portion 1 1 of the rivet hole 13 has a second diameter 3 8 which is equal to or larger than the diameter of the second rivet head 18. The gap portion 14 of the rivet hole 13 has a third diameter 15 which is constructed to provide an intermittent riveting tool and has a larger diameter than one end of the riveting tool. The second diameter 38 is larger than the first diameter 3 6 . The third diameter 15 is larger than the second diameter 38. Alternatively, the second diameter of the counterbore portion may be as large as the third diameter of the gap portion, but this may impair the strength of the sprocket which has a very open configuration in the region of the second rivet head. Figure 4 shows that the three largest sprockets 2d-2f are fixed to one of the carriers 6 1270619. A single rivet 7 is used to fix the largest sprocket 2f to the rear face of the carrier arm 6, and to fix the second large sprocket 2e to the front face of the carrier arm 6. The third large sprocket 2e and the other sprocket 2b-2d are respectively riveted to the end faces of the step portions 8 of the carrier arm 6. The sprocket 2a is riveted to the front face of the carrier arm 6. In order to mount the sprocket 2 to the carrier arm 6, the rivet shaft 17 of the rivet 7 is inserted into the rivet hole 24 from the rear face of the carrier arm 6. The first rivet head 16 is partially embedded in the counterbore portion 40 of the rivet hole 24 behind the carrier arm 6, causing only a small portion of the rivet head 16 to extend beyond the substantially flat surface behind the carrier arm 6. The second rivet head 18 is formed by a riveting tool at one end of the rivet shaft 17 extending in front of the carrier arm 6. During the riveting operation, the rivet shaft 17 expands and compresses the rivet holes 13 of the sprocket 2 and presses against a portion of the rivet holes 24 of the carrier arm 6. This is not only between the rivet 7 and the sprocket 2, but also forms a frontal connection between the rivet 7 and the sprocket carrier 3. This allows the sprocket 2 to be fixed to the sprocket carrier 3 in a non-movable and aligned manner. The second rivet head 18 is partially housed in the counterbore portion 1 1 of the rivet hole 13 and does not extend beyond the front surface of the sprocket carrier 3. The gap portion 14 of the rivet hole 13 prevents the rivet tool from directly hitting the sprocket 2 during the riveting operation. ® Figure 5 is another embodiment of the present invention, similar to the embodiment of Figures 1-4, except that the rivet 7 is replaced by a pin-type rivet 19. The first rivet head 30 on the rear surface of the sprocket carrier 3 and the second rivet head 32 on the front surface of the sprocket carrier 3 are formed by a caulking tool. During the riveting operation, the rivet shaft 34 of the pin-type rivet 19 expands and presses the rivet hole 13 of the sprocket 2 and presses against a portion of the rivet hole 24 of the carrier arm 6, not only at the rivet 19 and the sprocket 2e and 2f A front connection is also formed between the rivet 19 and the carrier arm 6. This allows the maximum sprocket 2f and the second large sprocket 2e to be fixed to the sprocket carrier 3 in a non-movable and aligned manner. -10- 1270619; The second rivet head 32 is partially housed in the counterbore portion 1 1 of the rivet hole 13 and does not extend beyond the front face of the sprocket 2 . The gap portion 14 prevents the riveting tool from directly striking the sprocket 2 during the riveting operation. As shown in these figures, the damage of the rivet tool to the sprocket is prevented by the gap around the rivet head; and the weight of the sprocket assembly is the open structure of the sprocket, the thin wall thickness of the hub, and the narrowness of the carrier arm. Reduced by the chemical. While the invention has been described with respect to the embodiments of the embodiments of the invention Therefore, the invention is not intended to be limited by the disclosed embodiments, but the scope of the inventions BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective front view of a sprocket assembly according to an embodiment of the present invention; Fig. 2 is a perspective rear view of the sprocket assembly of Fig. 1; Fig. 3 is a first view 3D front view of the sprocket assembly; Fig. 4 is a partial cross-sectional view of the sprocket assembly of Fig. 1, specifically showing the rivet mounting the sprocket to a sprocket carrier; and Fig. 5 is A partial cross-sectional view of a sprocket assembly of another embodiment of the invention. [Description of component symbols] 1... sprocket assembly 2a-2f... sprocket 3 ... sprocket carrier 4 ... hub -11- 1270619 . 5 ... inner key _ 6... carrier arm 7 .. rivet 8...step 10:. mounting bracket 11... countersunk hole 12···cutting • 13... rivet hole® 14... gap portion 14.. rivet hole gap portion 15. 3rd diameter 16...1st rivet head 17...rivet shaft 18...2nd rivet head 19...pin type rivet 20...sprocket hole β 22...sprocket tooth 24 .. . rivet hole 26 .. . chain Shaft rivet hole shaft portion 3 0... 1st nail head 3 2... 2nd rivet head 34.. Pin type rivet rivet shaft 36.. 1st diameter 38.. . 2nd diameter -12 1270619 40...burst hole of the arm rivet hole