Wu Jian Long

(KONE Elevator Co., Ltd., Kunshan 215300, Jiangsu, China)

Abstract:There are two connection modes between escalator step and step drive chain of outer roller, connector type A and type B. Connector type B after a period of running was found that the fixing bolt on the connector has loosened, which causes safety risks. In order to eliminate this security issue, the finite element modeling is proposed to analyze the relationship between the force generated by the connector type A or type B during the step band running and the assembly clearance or the assembly mode. So as to find out the root cause of fixing bolt loosening phenomenon, by redesigning the assembly clearance between the connector shaft and the chain hole, the fixing bolt of the connector is prevented from loosening during the step band running, ensuring the safe operation of escalators to avoid injury accidents.

Keywords: connector type; fixing bolt; loose reason; finite element analysis

1 Introduction

As an efficient transportation tool, escalator has been widely used in various places of public buildings. With the increase of its use, its security problems are becoming increasingly prominent[1-3]. Therefore, how to ensure the safe operation of escalator is very important. For heavy-duty escalators, the steps are running on tracks with four rollers supporting the step. One pair of these rollers, the chain rollers, is connected to the step chain with a cylindrical step connector, drive steps running. There are two different type of connectors, type A and type B. Lately some failures on connector type B have occurred and the question has arisen. To find the different design of the connector types is influential on this, therefore several finite element analyses of connector type A and type B are carried out.

2 Structure and types of connector

The components of the step connector are connecting shaft, drive chain of outer roller and step, the assembly dimensions and tolerances of connector type A and type B components are shown in Figure 1 and Figure 2, and the dimensions and tolerance of step assembly holes is shown in Figure 3.

Figure 1 Connection shaft and chain of connector type A

Figure 2 Connection shaft and chain of connector type B

Figure 3 Step assembly hole

2.1 Connector type A

The assembly structure of connector type A is shown in Figure 4. The bushing in the hole on one end of the shaft is assembled with the pin on the drive chain. The other end is assembled with the hole on the step and fixed together with bolts and holding plate.

Figure 4 Assembly structure of connector type A

2.2 Connector type B

The main difference with the connector type A is that the bushing of type B is inside the drive chain and the pin is on the connecting shaft. The assembly structure is shown in Figure 5.

Figure 5 Assembly structure of connector type B

3 Analysis on assembly clearance of connectors

The two ends of the connector shaft are assembled together with the drive chain and the step, as shown in Figure 6.

Figure 6 Assembly structure of connector shaft with drive chain and step

The assembly dimensions and tolerances of the connector type A and type B are shown in Table 1 and Table 2, and the calculated assembly clearance is shown in Table 3 and Table 4.

Two clearances: connector to step assembly (Step side) and connector to chain assembly (Chain side).

Table 1 Assembly dimensions and tolerances for connector type A

Table 2 Assembly dimensions and tolerances for connector type B

Table 3 Assembly clearance for connector type A

Table 4 Assembly clearance for connector type B

According to the data in Table 3 and Table 4, the assembly fit of the two types of connectors is as follows:

1) All clearances are clearance fits.

2) The clearance on the “Step side” has not changed significantly for connector type A and type B

3) The minimal clearance of connector type B on the “Chain side” has decreased significantly, and the clearance is 0.01 mm.

4 Finite element model and load cases

The step band load cases model is shown in Figure 7.

Figure 7 Step band load cases model

Materials steel:

Young′s modulusE=210 kN/mm2

Poisson′s ratiov=0.3

Densityρ=7.85 kg/dm3

Materials Aluminum:

Young′s ModulusE=70 kN/mm2

Poisson′s ratiov=0.3

Densityρ=2.7 kg/dm3

According to the assembly clearance of “step side” and “chain side” of the connectors, a finite element model is set up to model the different clearances[4-7]. Modeling of connector type A and type B, clearance fits are modeled by imposing offset on contact elements, Loads have been calculated for escalator with vertical rise of 20 m, step width 1 m. The load on each step was applied 250 mm out of the center towards the left side, therefore there is a load variation in left and right side.

Four different load cases are considered[8]as shown in Table 5.

Table 5 Load cases of connector type A and type B

5 Interpretation of results

Comparison between the models has not shown significant differences regarding overall deflection. However, the pressure in the chain side of connector Type B is significantly higher. The contact pressure for LC2 is shown as Figure 8 and Figure 9.

Figure 8 The contact pressure for LC2 (left side)

Figure 9 The contact pressure for LC2 (right side)

This higher pressure results from becoming a structural part of the chain in cases, where the chain is highly loaded. The effect of the chain can be clearly identified. The effect is sensitive to the clearance. In cases of relative motion between connector and chain, the contact pressure leads to a frictional torque on the connector. This relative motion occurs at the transitions between inclination and horizontal track and at the chain sprocket entry and exit. The frictional torque on the connector is calculated by integrating the contact pressure over the surface and multiplying it with an assumed friction coefficient of 0.1 and radius of the bushing, as shown in following.

The contact pressure over surface of the bushing is shown in Figure 10.

Figure 10 The contact pressure over surface of the bushing

An analytical estimation according to the Ref.[9] shows a minimal transferable torque via the frictional contact is about 37 N·m, and all friction coefficients is 0.12, the tightening factor is 1.6 (accounts for assembly uncertainty).

Lateral forces from are frictional torque in contact areas between connector/step and holding plate/step (perpendicular to drawing plane) and frictional torque is induced from relative motion between chain assembly and step connector, as shown in Figure 11.

Figure 11 Frictional torque and lateral force

The green line shows the minimal torque (37 N·m) transferrable via the bolted contact between connector and step.

The torque induced by friction in the bushing should be lower than 37 N·m. If a higher torque acts on this connection, there is a possibility of relative movement between those parts. Connector type A shows no significant values, connector type B exceeds the limit in several load cases. This effect is sensitive to the clearance, as shown in Figure 12.

Figure 12 Friction torques chart of connectors

6 Further analysis

To evaluate possible improvements, a new clearance was defined by changing the connectors diameter tolerances, and analyzed by finite element modeling. Shown in Figure 13, Table 6 and Table 7.

Figure 13 New tolerances for connector type B “chain side”

Table 6 Assembly dimensions and new tolerances for connector type B “chain side”

Table 7 New assembly clearance for connector type B “chain side”

To estimate the performance of higher clearances a simple finite element model of the chain was derived from the original to estimate the load bearing ratio of the connector pin inside the chain assembly. As shown in Figure 14, this model only consists of one side of the chain and one step connector Type B. All contacts between chain and chain links are modeled. Additionally a contact between the step connector and the bushing inside the chain assembly is modeled. Boundary conditions include a fixation of the chain at the lower end. The step connector is fixed at its end inz-direction (lateral to the chain). In order to assure numerical convergence, weak springs (c=20 N/mm) are added to allow for nearly force free movement in thex-y-plane. Because of the small stiffness, no relevant effect on the finite element results are expected. The load steps are considered: LC1 with 8 998 N chain load (empty) and LC2 with 51 224 N chain load (fully loaded). These correspond to above load cases for the left sided chain. The contact between step connector and bushing is modeled with a clearance of 0.01 mm, 0.083 mm and 0.153 mm. Plots of the von-Mises equivalent stress show the expected behavior, no load transfer to the step connector, as shown in Figure 15.

Figure 14 Force model of one side chain and one side connector type B

Figure 15 The equivalent stress diagram

The following plots show the contact pressure and contact status of different clearances for the simple models (Connector type B). As shown in Figure 16 and Figure 17, according to these, the pin with the higher clearance does not have a supporting effect on the chain, thus it is unlikely to get stuck and transfer a frictional torque in relative motion.

Figure 16 LC2 contact status of connector type B with different clearance

Figure 17 LC2 contact pressure of connector type B with different clearance

The results show that by changing the fit clearance between the type B connector pin and the chain bushing, the loosening of the fixing bolt can be solved, so as to avoid the potential safety risks.

According to literature[1-2], accidents caused by damage of steps, connectors and driving chains accounts for about 6% to 8% of the total escalator safety accidents. This research can reduce the safety accident rate of more than 90% due to the loosening of the connector bolt, which leads to the damage of the step, the connector and the drive chain and the injury or death of the escalator.

7 Conclusions

The main difference between connector Type A and Type B are in the position of the “chain side” bushing. While in Type A the bushing has a distance to the chain, in Type B it is located inside the chain assembly. The analysis shows that the bushing and connector stub of Type B are part of the load transfer of the chain. As the chain bushing and the step connector experience relative motion (rotation) when entering the chain sprocket or on the upper transition curve between inclined and horizontal movement, a torque on the connector resulting from friction in the bushing has to be considered. This torque is transferred via the frictional contact between connector and step to the step, where it is countered by the steps self-weight or support in the step rollers. The resulting moment induced from friction is significantly higher for Type B than Type A over all load cases. An estimation of the minimal transferrable lateral load on the frictional connection guideline shows, that it is exceeded by estimated occurring moments in some situations. If a bolted connection cannot transfer the lateral load by friction, there is the risk of relative moment of the bolt, where the holding plate and bolt move apart. Repeated occurrence of this effect might lead to a loss of pretension and the loosening of the bolt.

In conclusion, during the step band running, the contact pressure of the chain bushing to the connector generates a frictional torque greater than 37 N·m, which can cause the fixing bolt and the holding plate to loosen. A small assembly clearance between the chain bushing and the connector pin, and the bushing being located inside the chain assembly is the root cause of the high frictional torque.