EXTJS分页有关问题

求助：EXTJS分页问题
设置了分页bbar，且按钮可用，但是表格中数据并没有分页，列有286条记录，初始页面是1-286行，点击下一页就是51-286行
分页代码如下：
               var GridStoreZhuXing = Ext.create('Ext.data.Store', {
                    fields: json.ModelColumns,
                    data: json.GridData,
                    pageSize: 50,
                    proxy: {
                        type: 'memory'
                    }
                });

              GridStoreZhuXing.sync();

              GridStoreZhuXing.load({ 
                  params:{
                      start: 0,
                      limit: 50
                  },
                  callback: function (records, options, success) {
                      if (success) {
                          Ext.define("TempBiao10104VW", {
                              extend: 'Ext.grid.Panel',
                              alias: 'widget.xTempBiao10104VW',                                 
                              initComponent: function () {
                                  Ext.apply(this, {
                                      store: GridStoreZhuXing,
                                      layout: 'fit',
                                      id: 'TempBiao10104VWID',
                                      selModel: {
                                          //selType:'checkboxmodel'
                                      },
                                      border: 0,
                                      //multiSelect: true,
                                      frame: true,

                                      viewConfig: {